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Lee et al. Microstructures 2023;3:2023021 https://dx.doi.org/10.20517/microstructures.2023.08 Page 11 of 19

this sense, MOFs are strong candidates as a protective layer since they can provide a hydrophobic surface
and a uniformly tailored porous structure. A cesium lead bromide (CsPbBr ), an inorganic halide
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perovskite, has garnered interest as a viable photocatalyst for CO reduction. However, it shares similar
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limitations with other perovskites. Recently, researchers have made significant progress by employing
zeolitic imidazolate frameworks (ZIFs), a subfamily of MOFs, as a host matrix for CsPbBr quantum dots.
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Figure 7 illustrates the successful coating of CsPbBr QDs with ZIFs through a “building a bottle around a
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ship” approach. The mild synthesis conditions of MOFs enable the fabrication of QD-MOF composites in
this manner, providing reliable protection. The coated composite exhibits an improved CO reduction
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efficiency due to its superior capturing ability and charge separation efficiency, as well as enhanced stability
against moisture, compared to naked CsPbBr . However, this method may pose challenges in controlling
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QD particle size as the size is no longer restricted by the inherent pore structure of the template. Moreover,
the synthesis solvent may cause partial dissolution/decomposition of QDs, which can adversely affect the
overall efficiency. Thus, it is crucial to exercise caution in regulating the final solvent removal and
nucleation process .
[84]
Typically, the pores in MOFs are considered too small to accommodate QDs, as most reported MOFs have
microporous structures with pore diameters less than 2 nm. While this size range is suitable for the
adsorption and separation of small molecules, such as gases, it is not ideal for QDs which are typically
between 2 and 10 nm in size [85,86] . Synthesising QDs within the MOF cavity offers significant advantages in
controlling QD crystal size through MOF pores and limiting aggregation during fabrication. However, the
primary challenge lies in fabricating large-pore MOFs, as they can result in pore interpenetration and
blockage. A meticulous design of the pore structure is also crucial, with an ideal cavity having a narrow
[87]
entry to suppress the dissolution or dissociation of the encapsulated QDs . To address this limitation,
various efforts have been made to modify the pore sizes of MOFs to accommodate larger molecules such as
QDs. One such approach involves introducing larger functional groups into the organic ligands or metal
nodes, thereby expanding the pore size. Another approach is to employ post-synthesis modification
techniques, including solvent or ligand exchange, to adjust the pore size of the MOF. These strategies have
demonstrated promising outcomes in enlarging the pore size of MOFs, facilitating the integration of larger
guest molecules like QDs [88-90] .

In a recent advance, Qiao et al. reported the successful fabrication of CsPbBr quantum dots within the
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PCN-33 MOF pores through a sequential deposition method . PCN-333(Fe) possesses an extraordinary
[91]
hierarchically porous structure with mesoporous cages ranging from ca. 4 to 5 nm and microporous cages
with a diameter of around 1 nm, as well as excellent chemical stability. Its large mesoporous cages enable
the accommodation of perovskite nanoparticles, while its microporous cages facilitate the diffusion of
reactants for catalysis. In this study, PbBr was positioned within the PCN-333(Fe) cavity, followed by the
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diffusion of CsBr into the pores to form CsPbBr QDs within the mesoporous cage . This composite
[91]
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showed green fluorescence under ultraviolet (UV) light, which confirms the formation of CsPbBr QDs.
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Also, the resultant shows excellent stability, oxygen reduction, and evolving catalytic reaction in an aprotic
medium. As these photoelectrochemical characteristics are desired for photocathodes in lithium-oxygen
batteries, the material was tested as a cathode without any carbon support. The round-trip efficiency of the
battery under illumination was 92.7%, which was comparably higher than that of pure CsPbBr (82.8%) or
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PCN-333(Fe) (85.2%). Furthermore, it improved stability noticeably compared with pure CsPbBr as the
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MOF provides the protective aspect. This work shows the potential compatibility of perovskite QDs and
[91]
MOFs and also offers new promising directions and insights for photo-rechargeable batteries [Figure 8] .