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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 gcat−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 (IrOx) 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