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to methanol is stagnant. In the twenty‐first century, global warming is considered as a severe environmental problem, so ways of reducing CO₂ have been widely studied, including photoreduction. Wu [33] and Gondal and coworkers [34] tried to improve the photocatalytic conversion of CO₂ into methanol over traditional TiO₂ and modified ones by using novel light source, laser, and photoreactor, optical fiber, which to some degree enhanced the CO₂ conversion efficiencies and selectivity. However, the conversion rate was still at micromole level per photocatalyst weight hour, and by‐product H₂ accounted for large proportion of reductive products.

With the rapid development of nanotechnology and nanocatalyst, morphologies, surficial properties, and heterojunction have been extensively investigated in the past, which extremely promote the design and fabrication of highly efficient semiconductor‐based photocatalysts for CO₂ photoreduction. Zhang and coworkers synthesized Ag‐based plasmonic photocatalysts with different constitutions and shapes for enhanced reduction of CO₂ to methanol, where Ag nanoparticles can accumulate photogenerated electrons and benefit for reducing the potential barriers of CO₂ reduction [35]. Under visible‐light irradiation, with the assistance of anisotropic AgCl/Ag nanoparticles, the yield rate of methanol reached as high as 188 μmol g_cat⁻¹, and the activity of the photocatalysts has no obvious loss in the recycling reactions. Recently, Liu et al. prepared facet‐dependent Cu₂O that was coupled with reduced graphene, which was used to investigate the effects of Cu₂O‐exposed facet on the photoreduction of CO₂ to methanol [36]. The rhombic dodecahedral Cu₂O/rGO exhibited the highest methanol yield (355.3 μmol g_cat⁻¹), which was ca. 4.1–80.8 times superior to cubic octahedral Cu₂O/rGO and CuO/rGO after 20 hours of visible‐light illumination.

To further improve the yield and selectivity of methanol, Sun and coworkers prepared a single‐unit‐cell Bi₂WO₆ layers that exhibited enhanced CO₂ photoreduction because the density of states at the CB from the surface atomic layer is significantly increased upon reducing the 3D bulk Bi₂WO₆ to the atomically thin Bi₂WO₆ layers [37]. The reduced dimension is beneficial for rapid movement of the photogenerated electrons and holes to the surface and thereby improves the solar CO₂ reduction. What's more, the increased surface charge density can effectively facilitate the two‐dimensional (2D) conductivity, leading to the increase of CO₂ adsorption amount. Therefore, over single‐unit‐cell Bi₂WO₆, the predominant product of CO₂ photoreduction is methanol, and yield of CH₃OH reaches to 75 μmol h⁻¹ g⁻¹ under a 300 W Xe lamp irradiation, which is 125 times higher than that of bulk Bi₂WO₆ (0.6 μmol h⁻¹ g⁻¹). Meanwhile, Mi and coworkers revealed the potential of III‐nitride semiconductor nanostructures in solar‐powered reduction of CO₂ into hydrocarbon fuels [38]. It was demonstrated that CO₂ molecules can be spontaneously activated on the clean nonpolar surfaces of wurtzite metal nitrides, InGaN, based on the results of ab initio calculations, in which the photoreduction conversion rate of CO₂ into methanol is ∼0.5 mmol g_cat⁻¹ h⁻¹ under visible‐light illumination (>400 nm). Moreover, it was found that upon incorporating a small amount of Mg dopant into InGaN, the photocatalytic performance of CO₂ reduction can be drastically enhanced. It is well known that the separation and transport of photogenerated electrons and holes are largely governed by the presence of surface band bending, for instance, upward and downward band bending. For Mg‐doped InGaN nanowires, the introduction of Mg can make the surface potential of InGaN nanowires upward (up to 2 eV), leading to the near‐flat surface band structure, which can apparently improve the photocatalytic performance in CO₂ reduction.

In addition, in 2018, Gong and his group reported 0D/2D materials with polymeric C₃N₄ nanosheets and CdSe quantum dots to enhance the separation and reduce the diffusion length of charge carriers [39]. The rapid outflow of carriers also restrains self‐corrosion and consequently enhances the stability. Owing to quantum confinement effects of the QDs, the energy of the electrons could be adjusted to a level that inhibits the hydrogen evolution reaction (HER) (the main competitive reaction to the photocatalytic CO₂ reduction reaction) and improves the selectivity and activity for CH₃OH production from the photocatalytic CO₂ reduction reaction. With an optimal CdSe particle size of 2.2 nm, high selectivity (73%) and activity (186.4 mmol h⁻¹ g⁻¹) for CH₃OH production with an apparent quantum efficiency (AQE) of 0.91% are achieved, higher than most multicomponent CO₂ reduction reaction photocatalysts for CH₃OH production (generally 0.02–75 mmol h⁻¹ g⁻¹).

Besides, inspired by biocatalytic reaction, based on high selectivity of enzyme, a series of enzyme‐based inorganic/organic systems have been developed in the past. For example, Baeg and coworkers attached isatin–porphyrin to chemical‐converted graphene for constructing a newly developed photocatalyst that was used sequentially to couple with enzymes in the presence of nicotinamide cofactor (NADH) under visible‐light irradiation [40]. In this system, the selectivity of MeOH reached to almost 100%, while the yield (∼5 μM h⁻¹) is lower than that of efficient semiconductor‐based photocatalysts. Besides, in 2017, Park and coworkers further took advantage of enzyme‐cascade system to improve the transferring of photoinduced electrons into a multienzyme for biocatalyzed reduction of CO₂ to methanol in a photoelectrocatalytic system [41]. In this tandem photoelectronic cell with an integrated enzyme cascade (TPIEC) system, hematite, and bismuth ferrite were used as photoanode and photocathode, respectively, and NADH and three different enzymes (formate dehydrogenase, formaldehyde dehydrogenase, and alcohol dehydrogenase) as specific reaction enzyme catalyzed CO₂ into formate, formaldehyde, and finally methanol, as shown in Figure 2.10. The average rate of methanol formation per reaction volume (∼220 μM h⁻¹) is much higher than the pure photocatalytic system mentioned above.

(a) Schematic illustration of the solar‐assisted production of methanol from CO2 and water through a three‐enzyme cascade. (b) Methanol production as a function of time in multienzymatic systems. (c) Photoelectrochemical production of methanol by TPIEC under various applied potentials under visible‐light illumination. (d) Methanol production of the TPIEC system in various multienzymatic systems with an external voltage of 0.8 V

Figure 2.10 (a) Schematic illustration of the solar‐assisted production of methanol from CO₂ and water through a three‐enzyme cascade. (b) Methanol production as a function of time in multienzymatic systems. (c) Photoelectrochemical production of methanol by TPIEC under various applied potentials under visible‐light illumination. (d) Methanol production of the TPIEC system in various multienzymatic systems with an external voltage of 0.8 V.

Source: Kuk et al. [41].

2.4.1.3 Formaldehyde (HCHO)

As described in Eq. (2.3), HCHO is one of products that can be theoretically and experimentally formed, involving four‐electron reactions, while more elementary reactions happen during the final formation of HCHO [42]. Meanwhile, HCHO is also detected as an intermediate for fulfilling next reactions and producing CO and other advanced hydrocarbons. For example, Ojha et al. found that HCHO and COO– functioned as important intermediates to form CO over Cu‐modified S‐doped g‐C₃N₄ thin sheets through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [43]. Meanwhile, it is found that the existing forms of Cu in S‐doped g‐C₃N₄ system include Cu⁰, Cu⁺, and Cu²⁺ three states, which have obvious effects in the adsorbed species and reaction pathways. Owing to the higher absorbance value on the Cu⁰‐decorated sheets, the bicarbonate species as an intermediate tended to

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