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Li et al. Microstructures 2023;3:2023024 https://dx.doi.org/10.20517/microstructures.2023.09 Page 5 of 20

(5) The current atomically precise nano-chemistry is indeed moving toward the programmable synthesis of
MNCs with control over the structure, such as the bcc, fcc, hcp, decahedra, icosahedra, multi-tetrahedral
network, etc. (as shown in Figure 4) [45,46] . This programmable synthesis of MNCs offers promising
[47]
opportunities for designing their structures and enhancing their catalytic performance .

THE PCR MECHANISM OVER MNCS-BASED PHOTOCATALYSTS
PCR involves multielectron reduction, and various products can be obtained via different reduction
pathways, such as CO, CH , HCOOH, C H , HCHO, and CH OH. Below are the reduction potentials (E , V
2
4
3
4
0
vs. NHE) required for these products: [48]
CO + e → CO E = -1.900 V (1)
-
-
0
2
2
-
CO + 2H + 2e → HCOOH E = -0.610 V (2)
+
2
0
CO + 2H + 2e → CO + H O E = -0.530 V (3)
-
+
2
2
0
-
+
2CO + 2H + 2e → H C O E = -0.913 V (4)
0
2
4
2 2
CO + 4H + 4e → HCHO + H O E = -0.480 V (5)
+
-
0
2
2
-
CO + 6H + 6e → CH OH+ H O E = -0.380 V (6)
+
3
2
0
2
-
CO + 8H + 8e → CH + 2H O E = -0.240 V (7)
+
2
0
4
2
2CO + 12H + 12e → C H + 4H O E = -0.349 V (8)
-
+
2
2
2
4
0
-
+
2CO + 14H + 14e → C H + 4H O E = -0.270 V (9)
0
2
2
6
2
3CO + 18H + 18e → C H OH + 5H O E = -0.310 V (10)
+
-
2
0
2
3
7
The ultra-small size of NCs gives them a strong quantum size effect, exhibiting discrete energy levels that
allow electrons to undergo a leap from the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO), and electron-hole separation occurs. The energy gap of MNCs is
typically less than 2.2 eV, allowing photocatalytic reactions under visible light irradiation. Therefore, MNCs
can be considered semiconductor nanomaterials for photocatalytic reactions with small band gaps.
The mechanism of MNCs as catalysts in the PCR process is as follows (as shown in Figure 5) [49,50] . After
irradiating the MNCs-based catalyst with light with photon energy equal to or greater than the band gap
energy, the electrons transfer from the valence band (VB) to the conduction band (CB) to generate
photogenerated electrons and holes. The photogenerated holes are transferred to the surface active sites for
the oxidation reaction, as shown in process (1) of Figure 5 . In process (3) of Figure 5, photogenerated
[51]
electrons are transferred to the surface-active site for the reduction reaction. Therefore, the photocatalytic
performance is affected by the incident light absorption capacity and charge separation efficiency. To fully
utilize the energy from solar radiation, the band gap of the photocatalyst should be below 3.1 eV to
maximize light absorption . Most of the solar radiation energy reaching Earth is in the visible spectrum.
[52]
Additionally, the generated electrons and holes do not always migrate to the surface of the photocatalyst