From photosynthesis to photocatalysis: Dual catalytic oxidation/reduction in one system

Natural photosynthetic systems and photocatalysis share several fundamental processes in common including light energy conversion and utilization, such as exciton (excited state) generation/splitting and charge migration. The high efficiency of light conversion to chemical redox equivalents in natural photosynthesis is achieved by an electron transfer cascade resulting in a long-distance hole and electron separation across the membrane (1–9) (Fig. 1). Consequently, the charge geminate recombination is minimized, and the loss is reduced. The machinery of natural photosynthesis enables high-quantum yields of photon to electron/charge conversion efficiency facilitated by a fine-tuned local potential energy gradient by the protein surrounding each chlorophyll or its derivative to ensure unidirectional electron transfer and efficient final charge separation across the membrane (10–15). The holes and electrons separated far away from each other can thus go their separate ways to drive water oxidation and quinone reduction, respectively. Using the functions of metal oxide cluster or multiple reduction equivalents in molecules like quinones, natural photosynthesis succeeds in coupling multiple single-photon events with generation of multiple redox equivalents for catalytic reactions, such as water splitting.

Fig. 1.

(Left) Directional photoinduced electron transfer among bacterial chlorophylls and pheophytins in a sphaerodis bacterial reaction center protein results in charge separation across the membrane (not shown) with positive charges on one side and negative charges on the other side with reduced quinone derivative. (Right) Similarly, the Co₁C₃N₄AQ system by Chu et al. (22) also achieved directional electron and hole transfer resulting in two catalytic centers with Co to perform oxidative reaction and the AQ to perform reduction reaction in the same system. Right: Adapted from ref. 22, licensed under CC BY-NC-ND 4.0.

Inspired by natural photosynthesis, many artificial photosynthetic systems have been designed to achieve similar functions in photoinduced sequential, unidirectional, and long-distance electron transfer for simultaneous oxidation and reduction reactions at the both ends of the electron transfer chain (16–21). However, it has been a long struggle to effectively couple multiple single-photon events with multiple redox equivalent generation required for catalytic reaction such as water splitting. Frequently, only one of the two single half-reactions may take place, which not only uses only electrons or holes but also causes charge imbalance. Some systems may suffer the loss from charge recombination due to insufficient hole/electron separation after the exciton splitting and from the lack of directionality of charge separation. Thus, it has been a dream for photocatalysis or solar fuel community to design and synthetize platforms capable of accumulating both well-separated holes and electrons from photoinduced charge separation for sufficiently long time to carry out effective and simultaneous reduction and oxidation reactions in a single system.

The work presented by Chu et al. (22) titled “Spatially separating redox centers on 2D carbon nitride with cobalt single atom for photocatalytic H₂O₂ production” has shown commendable progress to prolong photoinduced hole/electron separation time and distance in a synthetic platform using C₃N₄ nanosheets that function as light-harvesting antenna and charge transport conduits. They cleverly designed two-dimensional (2D) C₃N₄ nanosheets to host a single-atom cobalt cocatalyst center at the voids in the middle of the sheet for the oxidative reactions and another cocatalyst anthraquinone (AQ) covalently linked at the edge of the nanosheets at least 1 nm away from the Co center for reductive reaction (Fig. 1). When the separation of hole and electron is right, these authors succeeded in carrying out photocatalytic reactions to generate H₂O₂ from O₂ and H₂O using the above platform and achieve multiple catalytic reactions in a single 2D C₃N₄ nanosheet.

Although this Co₁/AQ/C₃N₄ platform looks significantly simpler than the reaction center proteins in natural photosynthesis (Fig. 1), it has remarkably overcome the hurdle of charge recombination as exciton splits and achieved reaction selectivity. To confirm the separation of two different types of redox centers, a cobalt atom attached to a void in the middle as a water oxidation site, and an AQ attached to the edges of the sheets as a reduction site, these authors employed several physical characterization tools and obtained convincing evidence to support their results. Through the control of doping concentrations and locations of Co and AQ, they were able to set the two cocatalysts apart in a single nanosheet to prevent the loss due to charge recombination, and achieved unidirectional and distant electron transfer from Co site to AQ as well as scavenging both holes in the former and electrons in the latter, respectively, for the oxidative and reductive reactions in a single C₃N₄ nanosheet. Moreover, this Co₁/AQ/C₃N₄ system also enhanced catalytic reaction selectivity by using AQ toward H₂O₂ synthesis via two-electron reduction of O₂ (O₂ + 2H⁺ + 2e⁻ → H₂O₂) rather than four-electron reduction of O₂ (O₂ + 4H⁺ + 4e⁻ → 2H₂O) or two-electron H₂ evolution (2H⁺ + 2e⁻ → H₂).

In summary, Chu et al. in their study (22) simultaneously tackle the following challenges in photocatalysis in a platform of Co₁/AQ/C₃N₄: 1) moving hole and electron away from where they are generated via exciton splitting upon light absorption to minimize the loss due to geminate recombination via a semiconductor 2D C₃N₄ nanosheet; 2) segregating the photocatalytic oxidation cobalt site in the middle and reduction AQ site at the edge so that the two reactions can simultaneously take place without the needs for sacrificial donor/acceptor and external wiring; and 3) selecting proper AQ reduction site so that the reaction of forming H₂O₂ is competitive with other reactions in both energetic and kinetic standpoints.

Because of the above key advances, this research opens an approach for achieving both efficient oxidation and reduction reactions on 2D photocatalysts for many possible reactions that can be driven by renewable solar energy. We look forward to seeing more successful artificial photosynthetic systems and photocatalytic systems for fuel generation to store solar energy effectively.