**4. The photoreduction of CO2 applications of metal/semiconductor hybrid nanocrystals**

Photoreduction of CO2 is another important application of metal/semiconductor photocatalyst, and it directly converses CO2 into organic compound such methane, methanol, formalin, formic acid, and so on [125–130]. Being similar with photocatalysis for hydrogen evolution, metal/semiconductor photocatalysts for photoreduction of CO2 need some potential to meet the photocatalysis reaction (Schemes 1–5) [125],

$$\rm H\_2O \rightarrow H\_2 + 1/2 \, O\_{2'} \, \Delta E = 1.23 \, V \tag{1}$$

$$\text{HCO}\_2 + \text{H}\_2\text{O} \rightarrow \text{HCOOH} + 1/2\text{ O}\_2, \Delta \text{E} = 1.40\text{ V} \tag{2}$$

$$\text{HCO}\_2 + \text{H}\_2\text{O} \rightarrow \text{HCOH} + \text{O}\_{2'}\\ \text{AE} = 1.34 \text{ V} \tag{3}$$

$$\text{C} \text{O}\_2 + \text{H}\_2\text{O} \rightarrow \text{C} \text{H}\_3\text{OH} + 3\%\\2\text{ O}\_{2'}\text{AE} = 1.21\text{ V}\tag{4}$$

$$\text{CO}\_2 + \text{H}\_2\text{O} \rightarrow \text{CH4} + 2\text{ O}\_{2'}\\\Delta \text{E} = 1.06\text{ V} \tag{5}$$

and two pathways of photoreduction of CO2: formalin pathway and carbine pathway are proposed to explain the mechanism of photoreduction of CO2. According to these rules, various photocatalysts have been synthesized for conversing CO2 into fuel and modified for higher conversion efficiency. What is needed to point out here is that the reduction potential of CO2 depends on the reaction conditions, such as pH, and as the reduction is carried out, a series of reactions occur simultaneously. Therefore, as the reaction conditions vary, the main product of photoreduction of CO2 is different.

Nowadays, several series of semiconductor have been used for photoreduction of CO2, such as oxides [126, 131–133], sulfides [134], phosphide [135–136], and so on. In most case, metal will be used as cocatalysts to enhance photoreduction efficiency. Habisreutinger and his coworker have concluded photoreduction of CO2 using semiconductors (Table 1) and in most of the cases, metal would be used as cocatalyst [132]. However, as using different metal as cocatalysts, the main products of photoreduction of CO2 are different. It hints that metal nanocrystals play an important role in the photocatalysis system. It provides impossibility for tuning the photoreduction of CO2 by doping metal cocatalysts.

between metal and semiconductor constituents, the gradual symmetry evolution has led to novel control of optical response (plasmon–exciton coupling) and photocatalytic activity in hybrid nanostructures, which highlight the importance of nanoscale interface control for both fundamental understanding and technology applications. Thus, it can allow us to evaluate a possible upper limit of a photocatalytic reaction and guide the design toward high efficient hybrid nanostructures. Due to the plasmon intensity difference from different sized Au NPs, the larger sized Au, such as tens of nanometers to hundreds of nanometers size, the plasmon

**4. The photoreduction of CO2 applications of metal/semiconductor hybrid**

Photoreduction of CO2 is another important application of metal/semiconductor photocatalyst, and it directly converses CO2 into organic compound such methane, methanol, formalin, formic acid, and so on [125–130]. Being similar with photocatalysis for hydrogen evolution, metal/semiconductor photocatalysts for photoreduction of CO2 need some potential to meet

and two pathways of photoreduction of CO2: formalin pathway and carbine pathway are proposed to explain the mechanism of photoreduction of CO2. According to these rules, various photocatalysts have been synthesized for conversing CO2 into fuel and modified for higher conversion efficiency. What is needed to point out here is that the reduction potential of CO2 depends on the reaction conditions, such as pH, and as the reduction is carried out, a series of reactions occur simultaneously. Therefore, as the reaction conditions vary, the main

® D H O H + 1/2 O , E = 1.23V 22 2 (1)

® D CO + H O HCOOH + 1/2 O , E = 1.40 V 2 2 <sup>2</sup> (2)

® D CO + H O HCOH + O , E = 1.34 V 2 2 <sup>2</sup> (3)

® D CO + H O CH OH + 3/2 O , E = 1.21 V 22 3 <sup>2</sup> (4)

® D CO + H O CH4 + 2 O , E = 1.06 V 2 2 <sup>2</sup> (5)

enhancement for photocatalysis may become strong.

326 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

the photocatalysis reaction (Schemes 1–5) [125],

product of photoreduction of CO2 is different.

**nanocrystals**

As a kind of photocatalysts, TiO2 is one of the most popular semiconductors for photoreduction of CO2 [130]. For example, Li and her coworkers dope Cu onto TiO2 nanocrystals supported with mesoporous silica and enhance the photoreduction of CO2 (Figure 26A–D) [137]. More complex structure of photocatalysts has also been synthesized for photoreduction of CO2. Wang and his coworkers prepared the CdSe/Pt/TiO2 and monitor decrement of CO2 under irradiation of visible light (wavelength > 420 nm). They found that TiO2/Pt combining with CdSe platform a higher activity for photoreduction of CO2 (Figure 26E and F) [133].

**Figure 26.** Cu/TiO2 nanocrystals TEM (A) and HRTEM (B) images and its catalysis properties of photoreduction of CO2 (C and D). Copyright: Elsevier, 2010. CdSe/Pt/TiO2 nanocrystals and its photoreduction of CO2 (E and F). Copyright: America Chemistry Society, 2010.

Beside TiO2 nanocrystals photocatalyst, sulfides such as CdS nanocrystals and ZnS nanocrys‐ tals also show excellent properties for the photoreduction of CO2. It is reported that using ZnS nanocrystals as photocatalyst, the rate of formic acid production is up to 7000 μmol h-1 g-1 [132, 134]. What's more, the shape of nanocrystals and other factors on photoreduction of CO2 attract lots of attention and photoreduction of CO2 (Figure 27) [138].

**Figure 27.** SEM images and schematic of CeO2 modified with HPO4 ions (A–D) and CH4 evolution from photoreduc‐ tion of CO2 (E and F). Copyright: America Chemistry Society, 2015.
