A review of II–VI semiconductor nanoclusters for photocatalytic CO 2 conversion: synthesis, characterization, and mechanisms

The excessive consumption of fossil fuels has caused a severe energy shortage, and the large amount of CO 2 released during the combustion process has disrupted the carbon balance in nature. Achieving photocatalytic CO 2 reduction to high-value products is of high significance for both the economy and environment. So far, the bottlenecks for photocatalytic reduction of CO 2 include low electron–hole separation efficiency and low CO 2 productivity. II–VI semiconductor nanoclusters, especially magic-size clusters (MSCs), possess special chemical and physical properties such as an adjustable band gap (broadening the spectral response range), short carrier migration distance (favoring charge separation), and high surface-to-volume ratio (providing more active sites for CO 2 adsorption and conversion), making them potent candidates for photocatalysis. This review briefly introduces the research progress in II–VI MSCs. Then, we summarize the recent advances in II–VI MSCs and related composites for photocatalytic CO 2 reduction. Finally, the challenges and prospects of MSC-based photoelectron-catalytic systems are also discussed.


Introduction
0][11][12] Generally, an ideal material for photocatalytic CO 2 conversion should be stable and able to respond to a wide range of solar spectra for the generation of photoinduced electron-hole pairs with high efficiency. 9,12,130][21] In Liu et al.'s research, 22 magic-size (CdSe) 13 was stored at room temperature for a whole year to evaluate its stability.They found that (CdSe) 13 displayed better thermodynamic stability in comparison with other MSCs, and its morphology was maintained during the storage tests without any aggregation or decomposition.Besides, the band gap of CdSe nanoclusters can be accurately adjusted according to their size and surface ligand properties, thereby meeting the requirements of wide spectral responsiveness. 23Moreover, the small size and large specific surface area of semiconductor nanoclusters contribute to the exposure of catalytic active sites, making it easier for the combination and conversion of reactant molecules.Compared to the corresponding bulk materials, the average migration distance of photogenerated electron-hole pairs to the surface of MSCs is significantly shortened, which reduces their matrix recombination and increases the probability for them to initiate photoinduced redox reactions.Therefore, MSCs are very promising for the construction of photocatalytic systems with high performance, and they have met success in some pioneering works. 24,25owever, there are still several challenges to using II-VI MSCs for photocatalytic CO 2 conversion.On the one hand, the high dissociation energy of CQO in CO 2 (about 750 kJ mol À1 ) means that improving the activation ability of MSCs for CO 2 molecules is crucial to enabling efficient photocatalytic reactions over a wide spectral range. 26On the other hand, the tunable band structure of MSCs still needs to be optimized to make the conductive band (CB) more negative than the reduction potential of CO 2 , as shown in Table 1. 27The stability of MSCs in the appropriate solvent for CO 2 is another key point that contributes to the adsorption and conversion of CO 2 on their surface.9][30] It is reasonable to believe that using II-VI MSCs with clear structure, larger specific surface area, and tunable energy band as a substitute will significantly improve photocatalytic performance, although the related research is still rare.
The composition, morphology, and surface ligands of MSCs can be controlled through rational design and targeted synthesis strategies, which are key factors determining their optoelectronic properties and may have significant influence on their potential photocatalytic performance.Therefore, in this review, we first briefly overview the synthesis and compositions, growth mechanisms, and performance improvement strategies of typical II-VI MSCs.Then, recent advances on the application of II-VI MSCs for photocatalytic CO 2 reduction are introduced.Although there are still few reports on the relevant experimental progress, computational simulations predict the feasibility and bright future of MSCs in this field.Finally, the current challenges and future prospects of MSC-based photoelectron-catalytic systems are also discussed.

Synthesis and growth mechanism of typical II-VI MSCs
3][34][35][36][37] As is known, these metal chalcogenide nanomaterials with great potential in the fields of optoelectronic manufacturing, 16,38,39 photo/photothermal catalysis, 17,18 and new energy 40,41 have been studied in depth and seen great progress in the past few decades.In contrast, current topics are more focused on their ultrasmall intermediates due to the quantum confinement that brings about special phenomena and significantly enhances the conventional performance.
In relevant research, the first issue that needs to be addressed is the synthesis of MSCs.[36][37][42][43][44] Since the successful synthesis of (CdSe) 33,34 clusters by Kasuya et al. in 2004, 44 numerous direct-synthesized MSCs as well as other synthetic methodologies have been reported.Zhou and colleagues are one of the first groups to  24  8 conduct the synthesis and separation of II-VI semiconductor nanoclusters, pioneering the synthesis of nine II-VI ''magic size'' inorganic semiconductor nanoclusters. 45,46mong them, the strategy (see Fig. 1(a)-(c)) proposed by Buhro et al. is regarded as a universal one for the synthesis of most (ME) n clusters (M: metal, E: chalcogenide). 47,48Typically, a metal-salt primary-amine bilayer is formed firstly as a template.After adding chalcogenide precursor into the reaction system, (ME) n cluster is generated and released from templates due to the ligand-exchange-induced unbundling process.On this basis, the as-obtained (CdS) 13 MSCs could act as a single precursor to form colloidal CdS nanoplatelets with a wurtzite structure. 49In addition, Yu et al. 50demonstrated for the first time how colloidal semiconductor MSCs undergo isomerization at low temperatures under external chemical stimuli, namely, MSC-399 to MSC-422.Notably, ion exchange makes it possible for as-obtained MSCs to transform into their counterparts, indicating that new MSCs can be synthesized via a template-based approach (see Fig. 1(d)).
Precursor compounds (PCs) also play a vital role in the formation of MSCs.As reported by Yang et al., two different MSCs (MSC-299 and MSC-328) were transformed from ZnSe PCs by introducing octylamine and acetic acid into the reaction system at room temperature.Their study proposes the composition of PCs for the first time (ZnSe PCs with a formula of Zn 16 Se 32 ) and determines the formation of the covalent bond Zn-Se in ZnSe PCs, therefore deepening the understanding of the transformation relationship between PCs and MSCs and providing a new path for the synthesis of MSCs.In addition, diphenylphosphine (DPP) can inhibit the decomposition of Zn precursors and reduce the generation of by-products, which is also considered to play a crucial role in the synthesis of other PCs and related MSCs.
It is worth noting that MSCs with special stability always require ''jumps'' to obtain a series of crystallites with no intermediate sizes.In order to further clarify the growth mechanism of MSCs, nine magic-sized CdSe clusters with increased sizes were synthesized by Mule and coworkers, together with the proposition of a microscopic model based on classical nucleation theory. 34Results show that these nanocrystals grow layer by layer from one size to the next-larger size sequentially, which could be ascribed to the synergism of sizedependent nanoscale effects and the tetrahedral shape, while the surface-reaction-limited conditions take charge.Although this study only focuses on the growth of CdSe MSCs, the general growth mechanism is valuable for the study of related II-VI semiconductor MSCs and can provide guidance for their synthesis.

In situ characterization of typical II-VI MSCs
As is known, traditional non-in situ characterization techniques require a certain pre-treatment procedure for the test samples to meet testing requirements.During the process, some important information about their morphology, structure, as well as surface properties may be lost.Moreover, the detection of reaction intermediates is difficult to achieve using non-in situ characterization techniques.Therefore, researchers have turned to in situ characterization techniques for assistance, in order to monitor the reaction process in real time and speculate on the possible reaction mechanism by analyzing the intermediates in each stage of the reaction.To date, in situ characterization technologies run through the synthesis, separation, and performance evaluation of MSCs to determine their composition, structure, and physicochemical properties, while avoiding damage to them during the above process.Typically, time-of-flight mass spectrometry (TOF-MS) can provide exact information about the molecular mass and possible structures of MSCs by relating the molecular mass and the isotope patterns, which also helps to elucidate the formation mechanism of specific MSCs.As reported by Kasuya et al., highly stable mass-to-charge ratios (m/z) related to (CdSe) 33 and (CdSe) 34 were obtained by using laser desorption ionization mass spectroscopy (LDI-MS). 442][53] In addition, this relatively mature technique also contributes to determining the mesostructures formed by the assembly of MSCs. 546][57] According to Li's research, different Cd atoms (in the core or on the surface) can be differentiated by comparing the 113 Cd cross-polarization magic-angle spinning (CP-MAS) and 113 Cd MAS NMR spectra. 57he structure of MSCs is another current focus that can be determined by small-angle X-ray scattering (SAXS).Benefiting from the well-defined peaks, SAXS not only can characterize the crystal structure of MSCs but also demonstrate the existence of intermediates and mesophases during their formation, further elucidating their growth pathways. 58,59Moreover, the local environment, especially the structural changes induced by different capping ligands, can be investigated by extended X-ray absorption fine structure spectroscopy (EXAFS). 60bviously, through a series of in situ characterization techniques, the composition, structure, size, surface state, and growth mechanism of MSCs have been clearly revealed (see Table 2), providing valuable information for their future targeted design and modification to meet the needs of photocatalytic CO 2 reduction.

Modification on typical II-VI MSCs for enhanced performance
2][63] As reported by Hyeon, Mn x Cd 13Àx Se 13 nanoclusters (x = 0-2) with bright-orange photoluminescence (an evidence of Mn 2+ doping) were synthesized by taking a modified procedure applied for preparing the Mn 2+doped CdSe nanoribbons. 62The systemic introduction of Mn 2+ with controllable amount endows the production of MSCs with unique magneto-optical characteristics.This scalable method can also guide the synthesis of Co x Cd 13Àx Se 13 nanoclusters (x = 0-2), although there is a significant mismatch in the radii of Co 2+ and Cd 2+ . 63Notably, ion exchange makes it possible for as-obtained MSCs to transform into their counterparts, indicating that new MSCs can be synthesized via a template-based approach (see Fig. 1(d)).White et al. found that reorganization of the Cd 2+ sublattice in (CdSe) 33,34 initiated by Cu + doping (treating with tetrakis(acetonitrile) copper(I) hexafluorophosphate ([(CH 3 CN) 4 C-u]PF 6 ) in an oxygen-free and moisturefree glove box) created a meta-stable six-coordinate structure. 64The stronger affinity between this new structure and Cu + ensured continuous cation exchange, thereby successfully converting (CdSe) 33,34 into Cu 2 Se for the first time.As reported by T. Hyeon, 65 the CO 2 cycloaddition reaction (from propylene oxide to propylene carbonate) catalyzed by Mn 2+ :(Cd 0.5 Zn 0.5 Se) 13 MSCs suprastructures has a conversion rate of up to 95%.This new insight into the dynamics of ion exchange at the solid-liquid interface is a good supplement to the doping strategy, providing new ideas for the introduction of dopants into the MSC matrix with an accurate amount.
It should be pointed out that all the synthesis, modification, and performance improvement strategies mentioned above serve the potential applications of MSCs.Based on the scope  of this review, we will focus on the application of MSCs in the field of photocatalysis in the next section.

II-VI MSCs for photocatalytic CO 2 reduction
As mentioned earlier, the shorter carrier migration distance in MSCs increases the probability of photogenerated electronhole pairs transferring to the surface of the nanoclusters.On this basis, the charge separation efficiency of MSCs can be further improved by constructing heterostructures, thereby enhancing the photocatalytic performance of the nanocomposite.According to Choudhuri's calculations, encapsulating the CdSe nanocluster (Cd 6 Se 6 ) in NU-1000 (a metal-organic framework, MOF) allows the direct transfer of electrons (generated in the highest occupied crystal orbital [HOCO] on the linker of NU-1000 under visible light irradiation) to the lowest unoccupied crystal orbital (LUCO) of Cd 6 Se 6 (see Fig. 2), leading to effective charge separation and longer lifetime of the excited state, which is beneficial for photocatalytic reactions. 66Another important challenge faced by the chemical conversion of CO 2 is the activation process.Since structural irregularity at the atomic scale may endow small anionic metal clusters (M n À ) with superior catalytic performance, theoretical research was conducted by Lim et al. to investigate the anionic activation of CO 2 on these tiny nanostructures. 67Results based on density functional theory show that activated complexes (M n -CO 2 ) À were produced by the magic-numbered Cu, Ag, and Au clusters with 1, 2, and 6 atoms.Owing to the formation of M-C bond between M n À and CO 2 that is partially covalent, completely delocalized electrons were transferred from the HOMO of M n À to the LUMO of CO 2 , resulting in the chemical activation of CO 2 .Obviously, with the deepening of research on the synthesis of doped or alloyed MSCs through ion exchange, it is reasonable to believe that this strategy can effectively introduce the abovementioned metal species into MSCs to enhance their adsorption and activation ability for CO 2 , thus laying the foundation for high-performance photocatalytic CO 2 reduction.On this basis, a concept for efficiently selective reduction of CO 2 to CH 3 OH by the ternary composites (Cu N /MoS 2 /Ag(111), N = 1-8) was theoretically established, demonstrating the feasibility of MSC-included systems to convert CO 2 into valueadded chemicals. 68t is worth noting that although theoretical simulations predict the potential of MSCs in photocatalytic CO 2 reduction, there are still few reports on the relevant experimental progress.30][31]41 In our recent work, CdS/TiO 2 :Cu hollow spheres were constructed for photocatalytic CO 2 reduction with the assistance of H 2 S (see Fig. 3).The relatively high generation rates of CO (781.3 mmol g À1 h À1 ) and H 2 (5875.1 mmol g À1 h À1 ) indicate the high performance of the nanocomposite in the synergistic conversion of these two environmental hazards, as well as the huge potential for syngas production.Considering the enormous advantages of MSCs, it is highly anticipated that they can replace traditional II-VI nanostructures to achieve significantly enhanced performance in photocatalytic CO 2 reduction.

Conclusions
The advancement of synthesis and isolation methods, combined with modern characterization techniques, has given a deeper understanding of the composition, structure, and optoelectronic properties of II-VI MSCs.Moreover, the determination of growth mechanisms makes possible the rational design, controllable synthesis, and precise modification of II-VI MSCs  with customized performance.On this basis, the potential application of II-VI MSCs as a highly efficient photocatalyst was developed.
In view of the increasingly severe energy and environmental crises, as well as the steady progress of carbon peaking and carbon neutrality goals, it seems that the use of II-VI MSCs for photocatalytic CO 2 conversion can give full play to its advantages.In our opinion, progress still needs to be made in the following aspects before this can be realized: (1) A comprehensive consideration of various factors, such as mass production of high-purity MSCs, regulation of their spectral response range, stability in CO 2 -rich media, adsorption and activation capacity of CO 2 , and product selectivity is in great demand, which also points to the lack of experimental verification at this stage.
(2) It is worth noting that the main product of photocatalytic CO 2 reduction now is still CO.][71][72][73] (3) Combining the cathode with a high-efficiency photo anode to construct an all-light (solar)-driven self-biasing catalytic system, in which green and efficient CO 2 conversion without conventional energy consumption can be realized, is expected to become the ultimate goal in this field. 74,75t is worth noting that there are still many challenges to using II-VI MSCs for photocatalytic CO 2 reduction and constructing efficient CO 2 conversion systems driven by all-solar energy.However, with the continuous improvement of theoretical simulations, it is reasonable to believe that breakthroughs in related experimental research will be made in the near future.We hope that this review can inspire some constructive ideas and provide practical guidance and assistance for related work.

Fig. 1
Fig. 1 (a) The combination of metal salts and primary-amine solvents forms lamellar, primary-amine-bilayer templates.(b) The addition of group VI precursors results in the growth of magic-size nanoclusters within the templates.(c) Ligand exchange with oleylamine liberates sheetlike aggregates.(d) Formation of new MSCs via an anion exchange pathway.Reproduced from ref.48 and 49 with permission from the American Chemical Society.
Study the molecular mass, possible structures, surface ligands, growth pathways of MSCs Obtaining highly stable mass-to-charge ratios (m/z) related to (CdSe) structure, size and shape, and growth pathways (intermediates and mesophases) of MSCs Demonstrating the presence of intermediates formed during the induction period of CdTe MSCs 58-60 EXAFS Monitor the structural changes of MSCs induced by different capping ligands Discovering the relationship between Cd-S distance in CdS MSCs and their surface ligands (longer for thiol capping ligands, shorter for polyphosphonate capping ligands) © 2023 The Author(s).Published by the Royal Society of Chemistry EES Catal., 2023, 1, 687-694 | 691

Fig. 3
Fig. 3 (a) The complex scattering of light in the CdS/TiO 2 :Cu (CTC) hollow spheres.(b) Regeneration of the catalyst by heat treatment desulfurization.(c) The step-scheme charge transfer diagram between CdS and TiO 2 :Cu, as well as the redox reactions of the gaseous adsorbates under the full spectrum of xenon lamp.Reproduced from ref. 24 with permission from Wiley-VCH.

Table 1
Electrochemical potentials of H 2 O oxidation and CO 2 reduction into various products

Table 2
Study on the composition, structure, and surface properties of MSCs using in situ characterization techniques