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COMPLEXES OF SEMICONDUCTOR FRAGMENTS FOR SOLAR-LIGHT REDUCTION OF CO2 BY WATER
Ira Weinstock¹; Guanyun Zhang²; Josep Maria Poblet³; Chandan Tiwari⁴; Shubasis Roy⁵; Mark Baranov⁵;
¹BEN-GURION U. OF THE NEGEV, Beer Sheva, Israel; ²SHANDONG UNIVERSITY, Ji'nan, China; ³UNIVERSITAT ROVIRA I VIRGILI, Tarragona, Spain; ⁴CARESTREAM HEALTH INC., Saint Paul, United States; ⁵BEN-GURION UNIVERSITY OF THE NEGEV, Beer Sheva, Israel;
PAPER: 49/Nanomaterials/Regular (Oral) OS
SCHEDULED: 17:10/Wed. 29 Nov. 2023/Dreams 3

ABSTRACT:
While recent advances in functional materials increasingly involve the inclusion of metal-oxide domains [1], reproducibility problems inherent to their incorporation remain an often success-limiting challenge. In this context, molecular science could play a transformative role by using the tractability and versatile solution-state chemistries of well-defined molecular complexes to simplify device fabrication. This is demonstrated by using coordination complexes of structurally and electronically recognizable fragments of bulk metal oxides as versatile molecular "modules" for replacing the parent materials. For example, soluble hexaniobate complexed molecular fragments of cubic-spinel and monoclinic Co₃O₄ are highly active analogs of bulk cobalt oxide, with the HOMO and LUMO energies of the complexes, 1, closely matching those of the valence- and conduction-bands of the parent bulk oxides. Use of 1 as a tractable analog of cobalt-oxide nanocrystals is demonstrated by its deployment as a co-catalyst for the direct Z-scheme reduction of CO₂ by solar light and water [2]. Alternatively, complexed semiconductor cores can activate molecular polyoxoniobate cluster-anion ligands themselves as nucleophilic sites for CO₂ reduction. Although pure and functionalized solid-state polyniobates such as layered perovskites and niobate nanosheets are photocatalysts for renewable-energy processes [3], analogous reactions by molecular polyoxoniobates are nearly absent from the literature. Under simulated solar-light, however, hexaniobate cluster-anion encapsulated 30-Ni^II-ion "fragments" of surface-protonated cubic-phase-like NiO cores activate the hexaniobate ligands themselves. Photoexcitation of the NiO cores promotes charge-transfer reduction of Nb^V to Nb^IV, increasing electron density at bridging oxo atoms of Nb–&#m181;-O–Nb linkages that bind and convert CO₂ to CO. Photogenerated NiO “holes” simultaneously oxidize water to dioxygen. In related work, hexaniobate ligands are used to arrest the growth of metal-oxide NCs and stabilize them as water-soluble complexes. This is exemplified by hexaniobate-complexed 2.4-nm monoclinic-phase CuO NCs, whose ca. 350 Cu-atom cores feature quantum-confinement effects [4] that impart an unprecedented ability to catalyze visible-light water oxidation with no added photosensitizers or applied potentials, and at rates exceeding those of hematite NCs [5]. Together, the above findings point to polyoxoniobate-ligand entrapment as a potentially general method for harnessing the catalytic activities of semiconductor fragments as the cores of versatile, entirely-inorganic complexes.

References:
(1) Li, X.; Yu, J.; Jaroniec, M.; Chen, X. Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels. Chem. Rev. 2019, 119, 3962-4179.
(2) Zhang, W.; Mohamed, A. R.; Ong, W.-J. Angew. Chem. Int. Ed. 2020, 59, 22894-22915.
(3) Nishioka, S.; Hojo, K.; Xiao, L.; Gao, T.; Miseki, Y.; Yasuda, S.; Yokoi, T.; Sayama, K.; Mallouk, T. E.; Maeda, K., Science Advances, 2022, 8, eadc9115.
(4) M. A. Holmes, T. K. Townsend, F. E. Osterloh, Chem. Commun. 2012, 48, 371-373.
(5) S. Corby, R. R. Rao, L. Steier, J. R. Durrant, Nat. Rev. Mater. 2021, 6, 1136-1155.

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