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Li et al. Microstructures 2023;3:2023024 https://dx.doi.org/10.20517/microstructures.2023.09 Page 13 of 20

Figure 13. Schematic illustration of charge transfer in traditional Z-scheme heterojunction photocatalysts. D in the figure indicates
electron donors, and A in the figure indicates electron acceptors [94] . Copyright 2020, Elsevier Inc.

Figure 14. (A) A Z-scheme photocatalytic mechanism in natural photosynthesis system [90] . Copyright 2014, WILEY-VCH. (B)
Photogenerated electron transport rules between Au NCs and COF support [95] . Copyright 2020, Wiley-VCH.

In addition to Z-scheme heterojunction structures, S-scheme heterojunctions are utilized to fabricate
catalysts for improving PCR performance. (as shown in Figure 15A) . S-scheme heterojunction has the
[96]
following advantages: (1) the photocatalytic system can have both a wide photo-response range and a strong
redox ability; (2) the large internal contact area and the rapid separation of carriers in the S-scheme system
suppress the photo-induced electron-hole pair combination, which further improves the photocatalytic
ability [97-101] . Ke et al. initially fabricated a novel S-scheme SNO/CdSe-DET composite and investigated its
PCR activity (as shown in Figure 15B) . The SNO/CdSe-DET composites exhibited excellent CO
[102]
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photoreduction stability. Such a superior activity should be ascribed to the S-scheme system, which benefits
the separation of the photogenerated carriers and promotes the synergy between CdSe-DET nanorods and
SNO nanosheets by strong chemical-bonding coordination.

The encapsulation of MNCs in metal-organic frameworks (MOFs) has also garnered considerable
interest [103,104] . By encapsulating MNCs within the environment of the MOFs structure, their fluctuations and
aggregation can be effectively reduced, resulting in improved stability and catalytic efficiency. The MOFs
structure prevents MNCs from interacting with undesirable species in the reaction environment, which can
have a detrimental impact on their performance. Furthermore, the highly porous and interconnected
structure of MOFs enables the efficient mass transfer of molecules to and from the active sites of MNCs,
[105]
thereby enhancing their catalytic activity . Overall, the MOFs-encapsulated MNCs show enhanced
stability and improved performance, making them highly attractive for improving PCR performance.
Indrani Choudhuri and Donald G. Truhlar studied a composite material containing a Cd Se cluster in the
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pore of NU-1000 MOF. The Cd Se @NU-1000 composite permits electron transfer from the visible-light
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photo-excited organic linker to the lowest unoccupied orbital of the inorganic cluster, which can result in