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(^<i>i</i>pki)₂], <b>1</b>), bis­(4-(2-methoxyethylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(meki)₂], <b>2</b>), bis­(4-(2-ethoxyethylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(eeki)₂], <b>3</b>), bis­(4-(3-methoxy-propylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(mpki)₂], <b>4</b>), bis­(4-(3-ethoxypropylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(epki)₂], <b>5</b>), and bis­(4-(3-isopropoxypropylamino)­pent-3-en-2-onato)­cobalt­(II) ([Co­(^<i>i</i>ppki)₂], <b>6</b>) were synthesized and thoroughly characterized. Single-crystal X-ray diffraction (XRD) studies on compounds <b>1</b>–<b>3</b> revealed a monomeric structure with distorted tetrahedral coordination geometry. Owing to the promising thermal properties, metalorganic CVD of CoO_<i>x</i> was performed using compound <b>1</b> as a representative example. The thin films deposited on Si(100) consisted of the spinel-phase Co₃O₄ evidenced by XRD, Rutherford backscattering spectrometry/nuclear reaction analysis, and X-ray photoelectron spectroscopy. Photoelectrochemical water-splitting capabilities of spinel CoO_<i>x</i> films grown on fluorine-doped tin oxide (FTO) and TiO₂-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 TiO₂ underlayer, the CoO_<i>x</i> films exhibit photocurrents related to proton reduction under visible light