Aqueous Z-Scheme Photocatalytic CO₂ Reduction by Dispersed Semiconductor Particles of Metal Sulfide and Oxide in Synergy with Dual-Functional Metal-Complex

The photocatalytic carbon dioxide reduction reaction (CO₂RR), which can generate useful chemicals, is essential for realizing a carbon-neutral society and a circular economy.
To utilize visible-light energy and water as with natural photosynthesis, a 2-step photoexcitation mechanism (Z-scheme) that transfers electrons via two semiconductor photocatalysts is feasible [1]. Although an aqueous suspension of particulate photocatalysts in one-pot is an ultimately simplified and scalable approach, previous demonstrations have suffered from very low CO₂ selectivity against undesirable hydrogen production [1-3].
In this study, we demonstrate a simple activation methodology to overcome the issue by the combination of one molecule together with two semiconductor particulates in a water-filled one-pot reactor, achieving unparalleled selectivity of almost 100% of CO.
A simple mixture of two particulate semiconductors, a metal-sulfide, (CuGa)_0.3Zn_1.4S₂ as a CO₂ reduction photocatalyst (0.1 g) and a metal-oxide (BiVO₄) as a water oxidation photocatalyst (0.1 g) and a newly designed water-soluble molecular Co complex, [Co(4,4’-dimethyl-2,2’-bipyridine)₃]²⁺ ([Co-dmbpy]) were added to an aqueous solution of 10 mmol L^-1 NaHCO₃ (120 mL). Under CO₂ gas bubbling into the reactor and irradiation with visible light (λ > 420 nm), CO and a much smaller amount of hydrogen were continuously generated, while formate was almost undetectable. CO was generated with 98% product selectivity with respect to the total reduction products (CO, H₂, and HCOO^-) after 56 h. The CO generation rate of this system was increased by 1–2 orders of magnitude (38 μmol h^-1) relative to that obtained with a previously reported visible light-driven Z-scheme photocatalytic reaction using an aqueous suspension system, and was comparable to water splitting H₂ synthesis by the Z-scheme mechanism under similar conditions.
The simultaneous evolution of O₂ was confirmed, which clarified the extraction of electrons from water molecules with irradiated BiVO₄ necessary for CO₂RR in the Z-scheme mechanism. Experimental studies and calculations suggest that the Co complex acts dual-functionally in synergy with (CuGa)_0.3Zn_1.4S₂ and BiVO₄: it behaves as an efficient ionic electron mediator, and also acts as a new active CO₂RR cocatalyst after a structural change following the acceptance of photoexcited electrons from (CuGa)_0.3Zn_1.4S₂ [4, 5]. This simple method, operating in a self-optimizing manner in solution, has great potential to help achieve sustainable, highly active artificial photosynthetic systems.

References
[1] W. Zhang, et al., Angew. Chem. Int. Ed., 59 (2020) 22894-22915. [2] S. Yoshino, A. Kudo, et al., Acc. Chem. Res., 55 (2022) 966-977. [3] T. M. Suzuki, A. Kudo, T. Morikawa, et al., Chem. Commun., 54 (2018) 10199-10202. [4] T. M. Suzuki, A. Kudo, T. Morikawa, et al., Appl. Catal. B., 316 (2022) 121600. [5] T. M. Suzuki, A. Kudo, T. Morikawa, et al., Chem. Commun. 59 (2023) 12318-12321.