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Page 2 of 20 Li et al. Microstructures 2023;3:2023024 https://dx.doi.org/10.20517/microstructures.2023.09

Keywords: Photocatalysis, CO reduction, nanoclusters, photocatalyst
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INTRODUCTION
Fossil fuels have been the dominant source of energy for various applications such as production,
transportation, and power generation throughout human history. However, the excessive consumption of
fossil fuels has led to the massive emission of carbon dioxide (CO ), which poses serious threats to energy
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[1-9]
security and environmental quality . Numerous carbon fixation strategies have been developed to address
these challenges, such as CO emission reduction, CO capture and storage (CCS), and CO utilization [10-12] .
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CO emission reduction involves the use of innovative technologies to lower the amount of CO produced
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during the production stage. However, these methods are often associated with high costs that are difficult
for the general public to bear. The CCS also received significant attention, but the high cost and leakage risk
limit its application on a large scale. CO utilization, in contrast, is the most attractive path. Chemical
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reforming, electrochemical reduction, biological reduction, and photochemical reduction are the leading
CO utilization technologies. Photocatalytic CO reduction (PCR) is a sustainable process. It does not
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produce any toxic byproducts or cause any environmental pollution. PCR process can harness solar energy
to reduce CO and produce valuable energy sources such as methane or methanol. Moreover, the PCR
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process is usually operating under mild conditions of ambient temperature and pressure. These advantages
make PCR a highly desirable method for completing the carbon cycle [13-20] .
In the past few decades, the development of PCR catalysts has shifted from early bulky metals to
nanostructured materials with specific properties . Nanoclusters (NCs) are a new class of materials
[21]
comprising a few to several hundred atoms surrounded by ligands. NCs have three components: the inner
core, the outer atoms, and the surface ligands (as shown in Figure 1) . NCs exhibit distinct physical and
[22]
chemical properties compared to nanoparticles and molecules due to their distinctive electronic and
geometric structures. The majority of the current NCs are metal nanoclusters (MNCs) with sizes ranging
from 0.1 to 5 nm (typically < 2 nm for MNCs) , which is close to the Fermi wavelength (approximately
[23]
0.7 nm). Due to the quantum size effect, a single atomic change can drastically alter the electronic structure
and the physical and chemical properties of the clusters. Compared to conventional nanoparticles, MNCs
possess a much higher proportion of surface atoms. The surface atoms of MNCs have a low coordination
number, which results in high activity . Furthermore, MNCs are easier to synthesize than individual atoms
[24]
and also have a larger surface area and more catalytic sites than bulk materials .
[25]

Taking advantage of these inherent advantages, MNCs have been used as catalysts in various catalytic
[26]
processes. For example, Kurashige et al. investigated Au NCs as co-catalysts for H evolution .
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Gautam et al. presented a series of Au (GSH) NCs as co-catalysts on BaLa TiO for photocatalytic water
n
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[27]
splitting . Numerous efforts also have been devoted to the development of MNCs-based photocatalysts to
further improve PCR performance . For instance, Shoji et al. reported Cu O NCs as a general co-catalyst
[28]
x
and can be used in combination with various semiconductors to construct low-cost and efficient PCR
systems . Similarly, Gao et al. also reported Ag NCs as co-catalysts for selective CO photoreduction to
[29]
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CO . Additionally, Bo et al. bridged Au NCs with ultrathin nanosheets via ligands for enhancing charge
[30]
transfer in PCR . All these relevant reports above confirm that MNCs are promising catalyst candidates for
[31]
PCR.
However, the synthesis and utilization of MNCs for PCR remain challenging. It requires a comprehensive
understanding of their properties and catalytic reaction mechanisms. In this review, we provide a systematic
overview of the recent advances in MNCs-based catalysts for PCR applications. We first introduce a general