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Page 8 of 20 Li et al. Microstructures 2023;3:2023024 https://dx.doi.org/10.20517/microstructures.2023.09

Table 1. Performance of various noble & nonnoble MNCs used for PCR
Sample Performance References
Cu (II)-TiO 2 CO generation rate = 0.13 µmol/h; quantum efficiency = 27.7% [67]
2
Cu(II)-Nb-doped TiO CO generation rate = 0.20 µmol/h; quantum efficiency = 25.3% [69]
2 2
Cu(II)-TiO after the coordination of oxygen and metal CO generation rate = 0.40 µmol/h; quantum efficiency = 68.7% [75]
2 2
Fe(III)-TiO 2 CO generation rate = 0.40 µmol/h; quantum efficiency = 53.5% [77]
2
Fe(III)-Ti(IV)-TiO CO generation rate = 0.69 µmol/h; quantum efficiency = 92.2% [79]
2 2
-1
Ni/TiO CH CHO production rate = 1.42 µmol g-cat [81]
2 3
TiO /CoO x CO production rate = 1.2473 µmol/g/h; CH production rate = 0.0903 µmol/g/h [82]
4
2-x
/
CeO -S/ZnIn S CO productivity of 1.8 mmol/g with a rate of 0.18 mmol/g/h [83]
x 2 4
Au NCs/TiO /Ti C with CBD method CO yield of 27.5 µmol/g in 3 h; CH yield of 42.11 µmol/g in 3 h [87]
2
3 2
4
SNO/CdSe–DET CO production rate = 36.16 µmol/g/h [102]
Au-NC@UiO-68-NHC CO production rate = 57.57 µmol/g/h [107]
The initial focus of the research was to improve the visible light sensitivity of TiO semiconductors by using
2
Cu(II) or Fe(III) NCs as co-catalysts (as shown in Figure 6) [67,68] . However, this photocatalytic system has
limited catalytic efficiency because IFCT occurs only at the TiO /MNCs interface. Liu et al. designed a TiO
2
2
photocatalyst that can respond to visible light based on the principle of energy level matching . The energy
[69]
level occupied by the N-doped particles below the conduction band of TiO matches the potential of the
2
Cu /Cu redox couple in the Cu(II) NCs. The matched energy levels facilitate the efficient transfer of
2+
+
photogenerated electrons from the doped Nb state to the Cu(II) NCs, thereby contributing to the efficient
multielectron reduction of oxygen molecules (as shown in Figure 7) [69,70] . This method provides a practical
and strategic approach to creating new MNCs materials with effective photocatalytic properties.
Nonnoble MNCs-based catalysts improve PCR performance by increasing vacancies and defects
Nonnoble MNCs-based catalysts are not limited to simple grafting modifications as co-catalysts. It was
discovered that the performance of PCR could be enhanced by increasing the vacancies and defects. By
[71]
generating oxygen vacancies, these modifications increase the surface negative charge density . Upon
exposure to light, the oxygen vacancies accumulate additional negative charges that contribute to the
extension of the visible light absorption of semiconductor material, making the metal oxide capable of
activating CO .
[72]
2
Nolan et al. present a study of electron and hole localization in low-coordinated titanium and oxygen sites
of free and metal oxide-supported TiO nanocrystals (as shown in Figure 8A) . This approach highlights
[73]
2
how nonnoble MNCs can enhance oxygen and metal coordination by modifying semiconductor materials.
The structure of MNCs as catalysts in semiconductors reveals a significantly different metal and oxygen
coordination environment compared to that of the unmodified semiconductor [74,75] . Low-coordinated metal
and oxygen sites are crucial as charge carrier capture sites and active sites for target molecules, such as
carbon dioxide and water . Thus, Liu et al. developed a more sophisticated synthesis strategy by employing
[76]
MNCs with poorly coordinated metal and oxygen sites as catalysts [77,78] . Liu et al. also demonstrated that
amorphous Ti(IV) NCs promoted the oxidation of organic compounds effectively (as shown in Figure 8B)
[79]
and that TiO with Fe(III) and Ti(IV) NCs as catalysts achieved a Q.E. of 90% (as shown in Figure 8C) .
2
Additionally, Cheng et al. recently published the first study on Cu clusters mediated into Cd vacancies at the
[80]
edges of CdS nanorods for photocatalytic CO conversion . Billo et al. reported a Ni-NCs/TiO catalyst
2
2
with improved PCR performance . The Ni-NCs and O vacancies provide energetically stable CO binding
[81]
2
[81]
sites for CO reduction, allowing for rapid electron transport for enhanced solar energy harvesting . This
2
method enhances the photocatalytic activity and selectivity of Ni/TiO via a synergistic interaction in which
2