Rational Development of Cobalt β-Ketoiminate Complexes: Alternative Precursors for Vapor-Phase Deposition of Spinel Cobalt Oxide Photoelectrodes.

A series of six cobalt ketoiminates, of which one was previously reported but not explored as a chemical vapor deposition (CVD) precursor, namely, bis(4-(isopropylamino)pent-3-en-2-onato)cobalt(II) ([Co( pki)], 1), bis(4-(2-methoxyethylamino)pent-3-en-2-onato)cobalt(II) ([Co(meki)], 2), bis(4-(2-ethoxyethylamino)pent-3-en-2-onato)cobalt(II) ([Co(eeki)], 3), bis(4-(3-methoxy-propylamino)pent-3-en-2-onato)cobalt(II) ([Co(mpki)], 4), bis(4-(3-ethoxypropylamino)pent-3-en-2-onato)cobalt(II) ([Co(epki)], 5), and bis(4-(3-isopropoxypropylamino)pent-3-en-2-onato)cobalt(II) ([Co( ppki)], 6) were synthesized and thoroughly characterized. Single-crystal X-ray diffraction (XRD) studies on compounds 1-3 revealed a monomeric structure with distorted tetrahedral coordination geometry. Owing to the promising thermal properties, metalorganic CVD of CoO was performed using compound 1 as a representative example. The thin films deposited on Si(100) consisted of the spinel-phase CoO evidenced by XRD, Rutherford backscattering spectrometry/nuclear reaction analysis, and X-ray photoelectron spectroscopy. Photoelectrochemical water-splitting capabilities of spinel CoO films grown on fluorine-doped tin oxide (FTO) and TiO-coated FTO revealed that the films show p-type behavior with conduction band edge being estimated to -0.9 V versus reversible hydrogen electrode. With a thin TiO underlayer, the CoO films exhibit photocurrents related to proton reduction under visible light.