Solar-to-Chemical Conversion. Группа авторов

it is well demonstrated that layered nanomaterials with excellent conductivity, e.g. graphene and C₃N₄, can facilitate to accelerate the charge transportation, which is a promising component to efficiently improve the separation efficiency of photogenerated electrons and holes in traditional semiconducting photocatalysts [69]. For instance, Ye and coworkers constructed a smooth carrier channel in the basal plane of organic polymeric C₃N₄ photocatalyst by enriching defects with short and strong C–N chains through a glycine linker so that the photoinduced carrier transfer is apparently enhanced. Based on this, the photoreduction ability of CO is increased by 29.2 times under a simulated sunlight irradiation, as shown in Figure 2.13 [70]. Compared with complicated hydrocarbons, H₂ has strong competition ability to CO due to the lower potential. Therefore, H₂ often consumes more photoexcited electrons and suppresses the CO production. Although H₂ is cleaner energy storage form, carbon element is not contained in the energy cycle, which is undesirable to control CO₂ level in atmosphere. In other words, as a competitive reaction, two‐electron H₂ evolution reaction from water splitting should be suppressed deliberately. In 2011, Kudo and coworkers reported a series of Ag‐modified ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba) photocatalysts that have 3.79–3.85 eV of band gaps and layered perovskite structures, displaying photocatalytic activities of CO₂ reduction to produce CO and HCOOH without any sacrificial reagents [71]. It is reported that d⁰‐type metal oxide photocatalysts can catalyze water splitting into H₂ and O₂ because of high CB positions and wide band gaps, which are expected to be active for CO₂ photoreduction at a suitable reaction sites on the surface of photocatalysts. Among these photocatalysts, Ag‐loaded BaLa₄Ti₄O₁₅ showed the best photocatalytic activity. The Ag as a cocatalyst prepared by the liquid‐phase chemical reduction method was loaded as fine particles with the size smaller than 10 nm on the edge of the BaLa₄Ti₄O₁₅. On the optimized Ag/BaLa₄Ti₄O₁₅ photocatalyst, CO was the main reduction product rather than H₂ even in an aqueous medium. Furthermore, evolution of O₂ in a stoichiometric ratio (H₂ + CO:O₂ = 2 : 1 in a molar ratio) indicated that water was consumed as a reducing reagent (an electron donor) for the CO₂ reduction, which demonstrated that CO₂ reduction accompanied with water oxidation was achieved using the Ag/BaLa₄Ti₄O₁₅ photocatalyst. After that, Teramura and coworkers designed a new three‐component heterojunction for efficiently hampering the H₂ evolution and increasing the selectivity of CO. [72] The results indicated that Zn‐doped Ga₂O₃ exhibited significant restraint on H₂ releasing from overall water splitting. Based on this result, they further deposited Ag onto the Zn–Ga₂O₃ to increase the production efficiency of CO because introduction of Ag as a cocatalyst could enhance the harvesting efficiency of sunlight and collect more photogenerated electrons for CO₂ reduction. In the case of Ag/Zn/Ga₂O₃, the selectivity toward CO evolution is higher than toward H₂ in the presence of NaHCO₃ solution, where the yields of CO and H₂ reached to 800 and 60 μmol g_cat⁻¹, respectively, over seven hours under UV irradiation. At the same time, the O₂ evolution was observed, which inferred that overall water splitting happened in this system while H₂ releasing was suppressed. These results indicate that the Ag‐modified Zn‐doped Ga₂O₃ realizes selective conversion of CO₂ and H₂O to CO and O₂ under UV irradiation.

Figure 2.13 Structures of CN before (a) and after (b) doping with carbon chains. (c) Electrochemical impedance spectroscopy (EIS) Nyquist plots under irradiation condition. Inset: Periodic on/off photocurrent response under visible‐light irradiation. (d) Comparison of the photocatalytic CO₂ reduction rate of the samples with different amount of glycine, respectively.

      Source: Ren et al. [70].

      On the other hand, it is reported that CO can originate from the secondary photolysis of unstable reduced products, such as HCO₂H. For example, Frei and coworker investigated the reaction mechanism of CO₂ photoreduction over Ti silicalite molecular sieve in the presence of methanol as electron donor by means of in situ FTIR [73]. It was found that HCO₂H, CO, and HCO₂CH₃ as reduced products were detected, in which mass proportion of CO was the highest. The formation of the products was studied through the infrared analysis of experiments with isotope‐labeled reactants, such as C¹⁸O₂, ¹³CO₂, and ¹³CH₃OH. The results inferred that the produced CO is derived from secondary photolysis of the reduced HCO₂H. In contrast, the formic acid is the primary two‐electron reduction product of CO₂ at the ligand‐to‐metal charge transfer transition (LMCT) excited Ti centers. This means that since the complex hydrocarbons are the target products, the photolysis effect should be paid more attention for suppressing the formation of CO.

      2.4.3 Dioxygen (O₂)

As discussed above, an ideal artificial photosynthesis system usually contains at least three different components: light harvesting, water oxidation, and CO₂ reduction [74]. To achieve CO₂ photoreduction accompanied with water oxidation, early single integrated systems are composed of semiconducting metal oxide with large band gaps absorbing UV light and metal or metal oxide cocatalysts, where considerable efforts are paid to functionalizing these materials and exploring various modifications [75]. Nevertheless, since the assembly of multifunctional units in an integrated device is extremely difficult, an alternative strategy is to divide the overall process into two half‐reactions: water oxidation and CO₂ reduction. Once each half‐reaction is well understood and optimized, the two reactions can be coupled in an integrated device. For example, it is demonstrated that combining two semiconductor materials in tandem (Z‐scheme) mode can efficiently extend the light response into the visible region [76]. Recently, Arai et al. reported a photoelectrochemical system consisting of SrTiO₃ photoanode for water oxidation and InP photocathode for CO₂ reduction that produced O₂ and formate, respectively [77]. Besides, mononuclear [Ru(bpy)₃]²⁺ is often combined with S₂O₈²⁻ sacrificial acceptor to evaluate water oxidation photocatalysis systems in aqueous photosensitization system in the past decades [78]. For example, upon combining mild oxidant [Ru(bpy)]₃³⁺ of well‐defined potential (+1.26 V) with a Co₃O₄ nanotube in neutral aqueous solution, a hole can be efficiently transferred to the catalyst [79]. Subsequently, four holes transfer from [Ru(bpy)₃]³⁺ to Co₃O₄ and react with two H₂O molecules to O₂ and 4H⁺, which indicates that Co₃O₄ is appropriate candidate for use in an artificial photosynthetic assembly.

      On the water oxidation side, several groups proposed approaches to overcome challenges of efficiently combining molecular light absorbers with multi‐electron water oxidation catalyst inspired by the tyrosine mediator design of nature's photosystem II. For instance, the Mallouk group reported a series of researches on coupling of Ir oxide nanocluster (IrO_x) with [Ru(bpy)₃]²⁺ or a porphyrin visible‐light absorber through a benzimidazole–phenol redox linking with both components covalently anchored on a TiO₂ surface [80]. Based on the results of transient optical spectroscopy, when the Ru chromophore absorbs a visible photon, an photoinduced electron injects into TiO₂ at an ultrafast speed, and subsequently an electron transfers from the benzimidazole–phenol mediator to the oxidized Ru complex for reducing the oxidized chromophore