Enabling visible-light water photooxidation by coordinative incorporation of Co(II/III) cocatalytic sites into organic-inorganic hybrids: quantum chemical modeling and photoelectrochemical performance

Abstract

Coordinative incorporation of Co(II/III) cocatalytic sites into organic–inorganic hybrids of TiO₂ and “polyheptazine” (PH, poly(aminoimino)heptazine, melon, or “graphitic carbon nitride”) has been investigated both by quantum chemical calculations and experimental techniques. Specifically, density-functional theory (DFT) calculations (PBE/def2-TZVPP) suggest that Co(II/III) and Zn(II) ions adsorb in nanocavities at the surface of the hybrid PH–TiO₂ cluster, a prediction which can be further confirmed experimentally by ¹⁵N nuclear magnetic resonance in the case of the Zn complex. The absorption spectra of the complexes were characterized by time-dependent DFT calculations, suggesting a change of color upon Co ion binding which can in fact be observed with the naked eye. Hybrid TiO₂–PH photoelectrodes were impregnated with Co(II) ions from aqueous cobalt nitrate solutions. Optical absorption data suggest that Co(II) ions are predominantly present as single ions coordinated within the nitrogen cavities of TiO₂–PH, and any undesired blocking of light absorption is negligible. The cobalt-induced cocatalytic sites can efficiently couple to the holes photogenerated by visible light in TiO₂–PH, leading to complete oxidation of water to dioxygen. Our results indicate that coordinative incorporation of metal ions into well-designed surface sites in the light absorber is sufficient to drive complex multielectron transformations in artificial photosynthetic systems.

Keywords:

• Artificial photosynthesis
• Water photooxidation
• Photoanode
• Cobalt
• Oxygen evolution
• Time-dependent density-functional theory
• Hybrid materials

Acknowledgments

Computational resources were provided by the Centre for Research in Molecular Modeling (CERMM) and Calcul Québec. G.H.P. is a Concordia University Research Fellow. We thank the Sachtleben company for providing Hombikat UV 100. The support of the Center for Electrochemical Sciences (CES) is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

Dedicated to Professor Rudi van Eldik on the occasion of his 70th birthday.

¹ Aq and numbers hereafter refer to the number of water molecules.

² Initial and final systems ground state is singlet, but ΔE spin state change to quintet in the final state is only 3.5 kcal mol⁻¹.

³ As discussed below, singlet, triplet and quintet states for such system are pseudo-spin-degenerate, so spin-conservation rule might be more important in this case than the overall thermodynamics of the reaction including possible spin cross-over.

Additional information

Funding

This work was funded in part by the Ministére de l’éducation, du Loisir et du Sport du Québec and Concordia University. R.B., L.W. and D.M. are thankful for financial support by the MIWFT-NRW within the project “Anorganische Nanomaterialien für Anwendungen in der Photokatalyse: Wasseraufbereitung und Wasserstoffgewinnung” and by the EU-FP7 Grant “4G-PHOTOCAT” [grant number 309636].