Controlled growth of ultrafine metal nanoparticles mediated by solid supports

As a unique class of nanomaterials with a high surface-area-to-volume ratio and narrow size distribution, ultrafine metal nanoparticles (UMNPs) have shown exciting properties in many applications, particularly in the field of catalysis. Growing UMNPs in situ on solid supports enables precise control of the UMNP size, and the supports can effectively prevent the aggregation of UMNPs and maintain their high catalytic activity. In this review, we summarize the recent research progress in controlled growth of UMNPs using various solid supports and their applications in catalysis.


Introduction
Metal nanoparticles have fascinating properties applicable in many elds, such as catalysis, 1,2 sensing, 3,4 energy storage, 5,6 and medicine. 7,8 They have made signicant contributions to the development of heterogeneous catalysts, showing excellent catalytic activity and reusability. During the catalysis, the adsorption and desorption of substrates occur on the surface of metal nanoparticles. The partially lled d-orbitals can promote the adsorption of reactants and catalyze the product formation at active sites. Therefore, the number of surface metal atoms plays a critical role in the catalytic process. The smaller the diameter of metal nanoparticles, the larger the corresponding specic surface area and the larger the number of surface atoms (active sites). When the diameter of metal nanoparticles is reduced from 10 nm to 1 nm, the "effective" accessible metal atoms will increase from 20% to 90%. 9 Therefore, it is necessary to control the diameter of metal nanoparticles. Ultrane metal nanoparticles (UMNPs), which usually refer to metal nanoparticles with an average size of less than 3 nm, have shown great application potential in the eld of catalysis. 10 Typically, UMNPs have a high surface area and narrow size distribution, which can provide a large number of available active sites for heterogeneous catalysis. Having a large number of surface atoms, UMNPs are thermodynamically unstable. [11][12][13] Based on the migration-coalescence and Ostwald ripening mechanism, smaller particles aggregate to form larger particles through a spontaneous thermodynamic Hongyin Hu will receive his bachelor's degree from the School of Chemical and Material Engineering of Jiangnan University in 2021. He has obtained the exemption to study graduate qualications and will pursue his master's degree under the supervision of A.P. Shuanglong Lu. His current research interests focus on the synthesis of precious metal nanoparticles and their electrocatalytic application.
process. 14 The aggregation leads to reduced catalytic activity of UMNPs. Maintaining the dispersion and stability of UMNPs throughout the catalysis is also an important issue that must be solved to realize widespread practical applications of UMNPs in catalysis.
Supports and organic capping agents are two commonly used methods for the preparation of UMNPs and preventing their aggregation. 2,10,[15][16][17] Organic capping agents, such as oleylamine and polyvinyl pyrrolidone, are oen used to control the growth of metal nanoparticles. 16,17 However, these capping agents usually adhere to the surface of UMNPs through strong interactions, blocking most of the catalytically active sites, thus signicantly decreasing the catalytic activity of UMNPs. On the other hand, if the capping agent is removed, UMNPs tend to aggregate, which would affect their stability and also catalytic activity. Using solid supports to assist the UMNP synthesis can effectively solve the above problems. The supports can direct the growth of UMNPs and control their size through spatial connement and electronic effects, so that different UMNPs can be prepared by varying the types and structures of supports. 16,18 Moreover, by enhancing the interaction between MNPs and the support, the aggregation of MNPs can be minimized and the durability of the catalyst can be improved. Generally, metal ions or metal atoms whose dorbitals are not lled can bind with electron-rich atoms on the support, such as S, N, O, etc., through coordination bonds. [19][20][21] Therefore, heteroatoms or organic functional groups provided by supports are usually required for the growth and stabilization of UMNPs. In addition to the electronic effect, the connement effect of supports on UMNPs also makes a big difference. Particularly for porous materials with aligned or interconnected nanopores, the nanopores/ channels can conne the growth of metal nanoparticles. 22,23 Supports such as COFs, MOFs, and CMPs can be customdesigned and synthesized to achieve molecular-level control of the size of metal nanoparticles. [24][25][26] Simply varying the chemical nature of solid supports, a series of UMNPs with high activity, high surface area and narrow size distribution can be obtained.
In 2016, Xu et al. summarized the preparation of UMNPs immobilized on solid supports and their applications in catalysis. 10 They described the roles of different supports in detail and delivered a thoughtful review on the reported UMNPs mediated by solid supports. This review focuses on the solid supports (such as COFs, metal oxides/suldes, etc.) and the recent new research progress that has not been not covered previously. We discuss the controlled nucleation and growth of UMNPs using different solid supports, aiming to reveal the inuence of different supports (scaffolds) on the growth and performance of UMNPs. These studies have demonstrated that the solid supports can not only control the size of UMNPs and prevent their aggregation, but also improve their recovery and catalytic activity through electronic and spatial connement effects (Fig. 1).

The role of solid supports and design principles for UMNP preparation
UMNPs have a large number of available active sites and excellent catalytic properties, but they tend to aggregate aer cycles of catalytic reaction, resulting in a signicant decrease in the number of available active sites. Using a suitable solid support to anchor and isolate the UMNPs from one another is a feasible approach for maintaining the dispersion of UMNPs. The work of synthesizing metal nanoparticles using closed-shell structures as scaffolds and loading Pd nanoparticles in organic porous polymers (POP) has been systematically summarized. 9,27 The structural characteristics of the solid supports play an important role in the directed synthesis (e.g., size and morphology) and stabilization of the resulting UMNPs. Spatial connement and electronic connement have dominated the variations of structural characteristics in solid supports.

The electronic connement effects originated from the interaction between supports and UMNPs (or the metal ion precursor)
The nucleation and growth of UMNPs on the support are highly dependent on the nucleation site. The metal ion precursors will rst nucleate at the anchoring site, triggering the nanoparticle growth. 28,29 If the anchoring sites in solid supports are well distributed and provide strong interactions with the "seeds", the resulting UMNPs would remain highly dispersed during the synthesis and maintain uniformity throughout the catalysis process. There are three common ways to introduce nucleation and anchoring sites into supports: installing organic functional groups, heteroatom doping and creating defect sites. [30][31][32][33][34][35] Organic functional groups can bind the metal ions in the precursor solution. It is a common method to capture metal ions and anchor the resulting UMNPs in organic or hybrid supports, such as metal organic frameworks (MOFs) and covalent organic frameworks (COFs). 28,36 For example, graphene oxide is a carbon support with abundant oxygen-containing groups, which can act as nucleation and anchor sites for UMNPs. 37 Heteroatom doping changes the electronic properties of the supports and provides lone-pair electrons to form coordination bonds with the surface metal atoms of UMNPs. Suitable heteroatoms for anchoring UMNPs that have been reported include N, O, S, P and B atoms. [38][39][40][41] These heteroatoms may not only provide nucleation sites for UMNPs, but also improve the electron transport between UMNPs and the supports. Different heteroatoms are usually chosen for different UMNPs. Defect sites are also used as traps to capture UMNPs and prevent their aggregation. The defect sites in carbon materials, metal oxide/ sulde and other materials can be designed to control the growth of and anchor the UMNPs. 42,43 Embedding UMNPs in the solid supports could also enhance their surface area (thus accessible active sites) and improve their catalytic performance.

The spatial connement effects originating from porous supports
The diameter of UMNPs loaded on the porous supports is highly dependent on the size of the pores, which enables the conned growth of UMNPs and prevents their overgrowth. 25,44 By varying the diameter of the pores of the supports, the size of the UMNPs can be tuned. Such a connement effect of the porous support can also prevent the aggregation of UMNPs.

Synthesis of UMNPs supported by carbon materials
Carbon materials are the most economical and most widely used supports in both industrial applications and fundamental research. Typical carbon materials include graphene, diamond, porous carbon and carbon nanotubes. All of them exhibit good chemical stability, high conductivity and low density, which are all favorable features when used as supports for UMNPs. However, carbon materials oen lack sites for directly anchoring MNPs. Therefore, the use of carbon materials to support UMNPs oen requires surface modication or heteroatom (such as N, S, P, O, etc.) doping to introduce the functionality onto the carbon support, which provides anchoring sites for UMNPs. [38][39][40][41] At the same time, the doping of heteroatoms into the carbon materials can also effectively tune the properties of UMNPs anchored on the supports, further facilitating the catalytic reaction process and improving the catalytic efficiency.

Porous carbon
Porous carbon is an economical carbon material that has been mass-produced and widely used to support UMNPs. Porous carbon materials can be divided into microporous, mesoporous, and macroporous materials according to their pore size. According to International Union of Pure and Applied Chemistry (IUPAC) regulations, pores with diameters less than 2 nm are dened as micropores, those with diameters between 2 and 50 nm as mesopores, and those with diameters more than 50 nm as macropores. 18 The inherent pores of porous carbon can be used as the accommodation sites of UMNPs generated in situ and prevent their agglomeration, which is benecial to controlling the size of UMNPs. Microporous carbon usually has a large specic surface area, which can provide more anchoring sites for UMNPs. As shown in Fig. 2, Fan et al. coated microporous carbon bers with metal precursors and polyvinylpyrrolidone (PVP). 45 Metal-based nanoparticles/N-doped porous carbon hybrid lms were obtained upon pyrolysis. In this process, PVP was converted into a N-doped carbon thin lm, which provides binding interactions and connement effects for UMNP formation. Nearly uniform UMNPs (2 nm) were observed. Carbon bers were also etched by the metal at the high temperature to form micropores, increasing the specic surface area of the support with more exposed active centers. In addition, the nanoparticles are embedded in the carbon bers. With the additional protection of microporous carbon, the UMNPs showed good stability in electrocatalysis, even at high current densities. However, the narrow channels of microporous carbon materials and the strong interactions between the solid support and the product or reactant resulted in slow mass transfer. By contrast, macroporous carbon materials have spacious pore channels, which facilitate the transportation of the product or reactant. However, the specic surface area of macroporous carbon materials is usually low with relatively limited anchoring sites. Besides, the pores of macroporous carbon are too large to provide suitable connement for UMNPs. Therefore, porous carbon with only macropores is rarely reported as an efficient support for UMNPs. Mesoporous carbons have a relatively high specic surface area and spacious pore channels. Besides providing a large number of anchoring sites, they also possess relatively suitable pore channels for the mass transfer. Mesoporous carbons with ordered structures can be obtained by indirect template synthesis or direct synthesis. Yuan et al. have summarized the synthesis and modication methods of mesoporous carbons in detail. 46 Nitrogen-doped mesoporous carbons with abundant anchoring sites for UMNPs represent one of the most promising substrates for catalysis. Wu et al. used nitrogen-doped mesoporous carbons as substrate materials and obtained Fe and FeC UMNPs through low-temperature liquid phase precipitation and heat treatment, which show excellent performance towards the electrocatalytic oxygen reduction reaction (ORR). 12 In recent years, the carbonization of metal organic frameworks (MOFs) followed by chemical etching has emerged as one of the widely used approaches for preparation of mesoporous carbons. Zhang et al. prepared a mesoporous carbon support from the commercially available Cu-MOFs (HKUST-1) through the post-synthetic method. Such a solid support was used to prepared Pt UMNPs (2-3 nm), which have shown superior performance in both catalytic methanol oxidation and nitrophenol reduction reactions. 47 Beyond the two-step postsynthesis, MOFs can also serve as hosts for various precursors of UMNPs, following which one-step co-pyrolysis can be used to fabricate the UMNPs@mesoporous carbon hybrid material. Niu et al. prepared tungsten UMNPs supported on a MOF-derived mesoporous carbon through carbonizing ZIF-8 consisting of encapsulated K 5 BW 12 O 40 . 48 The catalysts show excellent performance towards olen epoxidation. It is worthy of noting that Zn volatilizes at high temperature, leaving the W species supported by porous carbon. Although the pyrolysis process will destroy the ne structure of MOFs, the mesoporous carbon support aer pyrolysis can still protect UMNPs from aggregation (Fig. 3).
To further optimize the electron and mass transfer during the catalytic process of UMNPs, hierarchical porous carbons, which contain two or three types of micropores, mesopores and macropores, are typically used. 49,50 Hierarchical porous carbons combine a high specic surface area and wide pore channels to provide abundant anchoring sites for UMNPs. At the same time, rapid mass transfer can still be achieved. Therefore, hierarchical porous carbons are very promising supports for loading UMNPs for applications in heterogeneous catalysis. Zhong et al. introduced ethylenediaminetetraacetic acid (EDTA) into MOF-808. 28 The EDTA acts as ion traps to capture Pt 2+ . The uncaptured Pt 2+ ions form PtO 2 clusters. Pt UMNPs loaded in porous carbon were obtained upon pyrolysis of the MOF precursors. With the increase of the Pt 2+ concentration from 10 ppm to 80 ppm, the diameter of the obtained Pt particles was increased from 2.1 nm to 4.1 nm. When the concentration was reduced to 2 ppm, the particle diameter did not decrease, but the distribution density of Pt particles was reduced. This series of Pt UMNPs showed low overpotential in catalyzing the hydrogen evolution reaction (HER).

Hollow carbon
Hollow carbon structures, such as carbon tubes and hollow carbon spheres, are commonly used as carbon materials for UMNPs. Their unique hollow structures can provide superior spatial arrangement of anchoring sites, thus maximizing the utilization of UMNPs. Carbon nanotubes (CNTs) are typical hollow carbon structures, which have a high degree of  graphitization, orderly arrangement of carbon atoms, excellent electrical conductivity and superior chemical stability. 51 These characteristics make them one of the most ideal support materials in various applications. The standing disadvantage of CNTs lies in their lack of anchoring sites for UMNPs. Therefore, CNTs oen need to be functionalized or doped with heteroatoms to improve their interactions with UMNPs. 52 There are many strategies for surface functionalization of CNTs, including oxidizing defect sites of CNTs, coating CNTs with polymers, and installing organic functional groups on the surface of CNTs covalently or non-covalently. [52][53][54][55][56] The surface functionalized CNTs have more anchoring sites for better stabilization of UMNPs. Simply linking functional groups to CNTs through chemical oxidation or covalent bonds is the most feasible method to assist the growth of UMNPs on the surface of CNTs. Liu et al. used KMnO 4 , concentrated sulfuric acid, and H 2 O 2 to oxidize CNTs in multiple steps, forming many defects and oxygen-containing groups on the surface of CNTs. 56 These groups increase the dispersion of CNTs in aqueous solution and assist the formation of Ag UMNPs ($3 nm) on the surface of CNTs. However, the oxidation process causes damage to the structure of CNTs and a signicant decrease in electrical conductivity, limiting their application in electrocatalysis. Chen et al. used ionic liquid polymers to uniformly introduce abundant functional groups onto the surface of CNTs through radical polymerization (Fig. 4a). 52 With the functionalized CNTs, uniformly distributed Pt (1.9 AE 0.5 nm) and PtRu (1.3 AE 0.4 nm) particles were successfully prepared on CNTs. The diameter of the particles loaded on non-functionalized CNTs was signicantly larger, and the size distribution was wider for both PtRu (3.5 AE 1.0 nm) and Pt (5.5 AE 1.5 nm) nanoparticles.
Similarly, as shown in Fig. 4b, Ma et al. used 3,4,9,10-perylene tetracarboxylic acid-derived ionic liquids to non-covalently functionalize CNTs. Pd 4 Au 1 -P nanoparticles (2.3 nm) supported on CNTs were obtained, which showed excellent ethanol oxidation (EOR) activity. 57 Since the functional groups were noncovalently connected to CNTs through supramolecular forces such as p-p stacking, the electronic structure of CNTs remained intact and the excellent electrical conductivity of CNTs was reserved.
Incorporating heteroatoms into the framework of CNTs is also a common strategy for introducing UMNPs anchoring sites into CNTs, particularly the introduction of N atoms. Wang et al. reported a template method to fabricate home-made carbon tubes with uniformly doped N atoms. 58 Different from conventional CNTs, these novel hollow carbon tubes are functionalized with a large number of binding sites for UMNPs. Highly dispersed Pd UMNPs (average size of 2.3 nm) were deposited on this N-doped carbon tube. Due to the synergistic effect of the support, Pd UMNPs can efficiently catalyze the reduction of 4nitrophenol even at a low loading (Pd: 0.324 wt%).
Although carbon tubes with hollow structures have been widely used as catalyst supports, most of the UMNPs supported by carbon tubes are usually anchored on the outer surface of carbon tubes instead of the internal surface. Huang et al. encapsulated a series of noble metal UMNPs (Pd, Pt, Ru and Au) inside open-end CNTs by wet impregnation, followed by pyrolysis (Fig. 5). 59 During the impregnation, metal ions enter CNTs through capillary action. When Pd UMNPs (2.5 AE 0.5 nm) were encapsulated into the CNTs, the resulting composite material showed excellent OER activity and can be used as a cathode material for Li-oxygen batteries. Other noble metal UMNPs, including Ru UMNPs (2.7 AE 0.5 nm), Pt UMNPs (1.8 AE 0.5 nm) and Au UMNPs (3.0 AE 0.5 nm) were also successfully encapsulated into CNTs.
Similar to carbon tubes, hollow carbon spheres (HCNs) with a unique hierarchical structure of a spherical shell represent another type of hollow carbon material, which also has highly exposed anchoring sites for UMNPs. Nanda et al. reported PtPd-UMNPs (2 nm) dispersed in heteroatom-doped HCNs, which showed excellent catalytic performance and stability in the methanol oxidation reaction (MOR), EOR and ORR. 60 The high  surface area of UMNPs brings about high catalytic activity, and the surrounding carbon layer prevents the migration and aggregation of UMNPs. The carbon structure also adsorbs a small amount of -OH/-H 2 O species, which is conducive to the removal of CO produced during the reaction, particularly in the MOR, to reduce the risk of catalyst poisoning. Fornasiero et al. reported a new synthetic approach to prepare FeO x UMNPs on the surface of HCNs through an "inside-out" mechanism. 61 They controlled the pyrolysis conditions, which can exsolve Fe species from the inside of the HCNs to the outside and form FeO x UMNPs. The different location (inside or outside) of FeO x led to the different selectivity (form H 2 O or H 2 O 2 ) in the ORR (Fig. 6).

Graphene and its derivatives
Graphene is a two-dimensional carbon nanomaterial with a hexagonal honeycomb lattice composed of carbon atoms and sp 2 hybrid orbitals. In theory, it has a huge specic surface area up to 2600 m 2 g À1 . Graphene has excellent optical, electrical, and mechanical properties and has important application prospects in materials science, micro-nano processing, energy, biomedicine, and drug delivery. 62,63 Graphene oxide (GO) is an important derivative obtained through oxidation of graphene. It has a large number of oxygen-containing functional groups, including hydroxyl, epoxy, carbonyl, carboxyl, etc., which are favorable anchoring sites for metal ions and UMNPs. Reduced graphene oxide (rGO) is obtained by reduction of GO. Compared with graphene, rGO not only has residual oxygen and other heteroatoms, but also has some defects, which can act as barriers for atom migration and maintain the uniform distribution of UMNPs. 64 Graphene only shows good dispersibility in non-polar solvents, which limits its use due to the fact that most metal precursors are dissolved in polar solvents. 64 In addition, the uniform structure of graphene lacks the anchoring sites for UMNPs. In contrast to graphene, GO can be easily dispersed in water or polar organic solvents. The oxygen-containing groups of GO can facilitate exfoliation of graphene nanosheets and prevent graphene stacking. GO can be easily produced at low cost, but strong oxidants are inevitably used in the GO production process, resulting in a large number of defects in the GO network. It causes the conductivity of GO to be far inferior to that of graphene. rGO exhibits conductive properties similar to graphene and contains a small amount of unreduced oxygencontaining functional groups and defect sites. 65 The most convenient way to anchor UMNPs on GO is to reduce the metal precursor and GO at the same time. Then, the metal precursor will be reduced to form UMNPs while GO transforms into rGO. Ravishankar et al. used ethylene glycol to simultaneously reduce Pt 2+ and GO under microwave irradiation conditions and load Pt UMNPs (2-3 nm) on graphene. 66 Liang et al. reported the formation of uniformly dispersed Cu UMNPs (2 nm) on rGO through laser ablation of a Cu target in GO solution followed by reduction treatment. 67 The obtained Cu UMNPs showed excellent performance towards 4-nitrophenol reduction. However, the reduction process of metal precursors also reduces the oxygen-containing functional groups on GO. To prevent GO stacking due to the loss of oxygen-containing functional groups, Jin et al. introduced spherical carbon black as the gap between GO layers to form a stable 3D structure and maintain the high specic surface area of GO (Fig. 7). 68 The carbon black and graphene doped with nitrogen were obtained upon reduction with urea. They used such a unique three-dimensional structure as the support for the growth of Pd UMNPs. Both the incorporated N sites and the initial oxygen-containing functional groups serve as the binding sites for Pd ions. Upon NaBH 4 reduction, Pd UNMPs (3.2 nm) grew in situ in the retained 3-dimensional composite structure of rGO and carbon black. The hybrid catalysts showed excellent activity towards the electrocatalytic ethanol oxidation reaction.

Synthesis of UMNPs supported by crystalline frameworks
Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) are emerging porous materials with rigid frameworks and orderly lattice fringes. Unlike conventional  crystalline materials, these framework materials are constructed from organic building blocks or through hybridization of organic building blocks and metal ions. By selecting different organic monomers, porous crystalline frameworks with different pore sizes, morphologies and functional groups can be synthesized. These materials typically have great structural tunability and high porosity, making them ideal catalyst support materials. [69][70][71] Apart from being precursors for porous carbon which has been discussed in Section 3.1, the rigid frameworks provide stable pores without collapse for a long time, and the backbones between the cavities can prevent the agglomeration of UMNPs.

MOFs
MOFs are a class of organic-inorganic hybrid materials with intramolecular pores formed by self-assembly of organic ligands and metal ions or clusters through coordination bonds. MOFs have highly tunable pore size and volume and an easily modiable inner surface. 69 In addition, their frame structure is rigid and not easy to collapse. Moreover, MOFs usually have good thermal stability. These exciting features make MOFs very promising support materials for UMNPs. Through rational design, metal ions or UMNPs can be precisely and uniformly coordinated and deposited on MOFs. 72 The pore structures of MOFs separate the UMNPs one from another, which can avoid the aggregation of UMNPs during the catalytic process. 73,74 The synthesis and application of MOFs is a current hot research eld, and many new MOF structures are reported every year, but not all MOFs are suitable for the synthesis of UMNPs and their application in catalysis. For example, when H 2 O is generated or participates in the catalytic reaction pathway, the framework of MOF-177 can easily decompose and collapse. 75 MOFs with suitable pore structures and high stability are required for the conned growth of UMNPs for catalysis application. Xu et al. have reviewed the controlled synthesis of UMNPs by using MOFs as supports in detail. 10 Therefore, we will only briey discuss the typical cases of the controlled synthesis of UMNPs with MOFs in recent years in this section.
Li et al. demonstrated a method of using MIL-101 as a template to x a variety of bimetal alloyed UMNPs in the pores (Fig. 8). 24 The particle diameter was 1.1-2.2 nm, and the metal loading was up to 10.4 wt%. Taking Cu-Pd@MIL-101 as an example, they rst introduced Cu 2+ into MIL-101 by dipping and then reduced them with NaBH 4 to obtain Cu particles with a diameter of 8.25 AE 2.53 nm. Subsequently, the Pd precursor was introduced, and under the action of high-intensity ultrasound, the Cu particles were effectively transformed into Pd-Cu UMNPs (diameter of about 2.16 nm) through the electric substitution reaction with Pd 2+ . The catalyst was used to catalyze the coupling of phenylacetylene to produce 1,4diphenylbuta-1,3-diyne in excellent yield (up to 98%).
The heteroatoms incorporated into the MOF backbone or side chains can act as nucleation and anchor sites just like carbon materials. The surface micro-environment of UMNPs can be modulated by changing the functional groups and metal substitution in the MOFs to enhance their catalytic performance. Jiang et al. encapsulated Pd UMNPs (less than 1.1 nm) into the pores of UiO-66-X (X ¼ H, OMe, NH 2 , 2OH, 2OH(Hf/Zr)): X represents the tunable organic groups linked to the benzene ring, Hf/Zr represents the Hf-oxo or the Zr-oxo cluster. 76 Different organic linked groups in the pores of the MOFs can change the adsorption energy of Pd@UiO-66-X to a substrate and the charge transfer between Pd UMNPs and MOFs. The catalytic activity of Pd@UiO-66-2OH was around 14 times higher than that of Pd@UiO-66 in the hydrogenation of benzoic acid. In addition, changing the metal clusters (2OH, 2OH(Hf)) also led to different catalytic performances. The authors highlighted the inuence of the surface microenvironment on the guest metal NPs, which can be tuned by choosing different substituents of the metal and functional groups in the host MOFs.
Embedding UMNPs into the lattice of MOFs can enhance the dispersibility of UMNPs. The electronic conguration at the interface between UMNPs and MOFs can be tuned, which may increase their catalytic performance. Zhou et al. used cobalt and benzimidazole to synthesize a MOF, (poly-[Co 2 (benzimidazole) 4 ] (PCB)), with a laminar ow structure and introduced Fe 3+ into the pores of the MOF by dipping (Fig. 9). 77 Aer reduction, the generated FeCoO x UMNPs (3 nm) were embedded into the lattice of MOFs. When deposited on carbon cloth, such a composite material showed excellent activity in catalyzing the oxygen evolution reaction (OER).

COFs
COFs are a class of 2D or 3D crystalline organic polymers connected by covalent bonds. Similar to MOFs, COFs also have highly tunable pore size and easily modied inner surface. Therefore, the introduction of suitable functional groups into the constituent units of COFs can equip COFs with binding sites for UMNPs. 78,79 Compared with MOFs, COFs have lower density without any metals in their frameworks. In addition, they generally exhibit higher stability in acid, alkali, organic and aqueous media when compared with MOFs.
The introduction of anchoring sites into COFs has been mainly accomplished in two ways: (1) incorporating the sites into the backbones of COFs and (2) installing the sites as side chains on the COF backbones. The N-rich COF backbones have a large number of N atoms, which can provide abundant anchoring sites for UMNPs. Through the coordination of N atoms and the connement of intrinsic pores, UMNPs can be well encapsulated and the aggregation of UMNPs can be prevented. A series of COFs with N-rich backbones have been successfully developed using synthetic building blocks with nitrogen-containing functional groups such as carbazole, porphyrin, amide and imine. Lu et al. used piperazine and cyanuric chloride as molecular building blocks to synthesize a COF with the N-rich backbone that can coordinate Ru ions (Fig. 10a). 80 Ru ions were reduced in situ to Ru UMNPs (1.4 to 2.6 nm), which were embedded in the pores of the COF. The connement effect from the pores and the coordination with N atoms rmly anchored the UMNPs in the COF framework. The obtained UMNPs were used to catalyze the methanolysis of ammonia borane, delivering a total turnover frequency (TOF) up to 505 min À1 at 298 K. Moreover, the catalyst showed excellent durability and maintained high dispersion during the catalytic process.
In addition to nitrogen-containing functional groups, other heteroatom-containing functional groups can also be used as anchor sites to coordinate UMNPs. Phosphine groups have been widely used as organic ligands for metal organic catalysts. Incorporating phosphine groups into COFs can make the phosphine groups distributed in an orderly manner and provide anchor sites for metal ions or UMNPs. Recently, a COF material (Phos-COF-1) that incorporates triphenylphosphine (PPh 3 ) as a growth site for UMNPs was reported. 33 A series of UMNPs (Pt, Pd, Au and PdAu) with narrow size distribution were successfully prepared inside Phos-COF-1 (Fig. 10b). The loading of metal precursors is also an important factor affecting the size of UMNPs. Excessive metal loading promotes the excessive random growth of UMNPs on the surface of COFs. Taking the Pd UMNPs synthesized in this work as an example, aer doubling the amount of the Pd precursor, the diameter of Pd UMNPs changed from 1.62 nm to 2.74 nm on average and the size distribution range became wider. The obtained catalysts were used to catalyze a number of reactions including nitrophenol reduction, cross coupling, and tandem cross couplingnitrophenol reduction. All of these catalysts exhibit excellent performance.
Besides directly anchoring UMNPs through the sites in the backbone of COFs, COFs can also be functionalized in their side chains to improve the binding ability towards metal ions or particles. Post synthesis and bottom-up synthesis are the two commonly used strategies to install functional groups. [81][82][83] The post synthesis avoids functional groups participating in the polymerization of building blocks to form COF backbones. However, through post-synthesis, the crystallinity of COFs may be reduced and not all pores can be fully functionalized. The advantage of bottom-up synthesis lies in the well-maintained crystallinity of COFs and the functional groups can be evenly distributed in the framework. However, some functional groups may either be unstable under severe synthetic conditions of COF formation or interfere with the formation of crystalline COF structures.
Recently, Lu et al. reported a COF containing thiol groups (-SH) via post-synthesis strategies (Fig. 11a). 84 The small pores of COFs and the strong S-Au binding energy enabled the conned growth of Au UMNPs with narrow size distribution (1.8 AE 0.2 nm). Such a composite material showed enhanced photostability and photocatalytic performance in the photodegradation of RhB. The synthesis of a COF containing thioethers (Thio-COF) using bottom-up strategies was also reported (Fig. 11b). 25 Pt UMNPs (1.70 nm) and Pd UMNPs (1.78 nm) conned in Thio-COF were obtained, which showed excellent catalytic activity towards the reduction of 4-nitrophenol and the Suzuki-Miyaura coupling reaction, respectively. The thioether group with strong binding capability for metal ions or nanoparticles was pre-installed in the building block. It  has been proposed the thioether groups serve as nucleation centers for the conned growth of UMNPs in COFs. When COFs without thioether in their channels were used as supports, randomly agglomerated metal particles were formed instead. In addition, the use of an amorphous COF with the same structure as the support also led to random distribution of particles. Therefore, both the crystalline structure of COFs and the suitable functional groups in the channels contribute to the high quality of the as-formed UMNPs. Similar studies using bottomup synthesis of functionalized COFs for UNMPs have also been reported for Pd UMNPs@COF, Fe-TiO 2 UMNPs@COF. 19,85

Synthesis of UMNPs supported by amorphous organic polymers
In addition to ordered COFs, there are some other disordered polymer materials, which can also be used as supports to direct UMNP growth. Their design concept is similar to COFs, that is, modifying the polymer backbone with organic functional groups or incorporating heteroatoms. 86 They can also provide a conned environment for the growth of UMNPs. Several common amorphous polymers that can be used to control the synthesis of UMNPs include conjugated micro-/meso-/macroporous polymers (CMPs), hyper-crosslinked polymer (HCPs) and other porous organic polymers (POPs).
CMPs have p-conjugated main chains and permanent porous structures, which distinguishes them from unstable porous materials, non-porous conjugated polymers, and porous carbon materials. By choosing different molecular building blocks, CMPs with different pore sizes and morphologies can be obtained. 87 It is worth noting that because imine bonds can contribute to conjugation, certain imine-based COFs can also be considered as crystalline CMPs. CMPs can also be considered amorphous analogs of COFs. 88 The conjugated structures of CMPs improve the electron transfer efficiency, which helps in electron transport in the catalysis process. It is even anticipated that it could be used directly in electrochemical catalysis. The work of Maji et al. gave an example of loading UMNPs onto CMPs and directly using them in the electrochemical catalysis (Fig. 12a). 89 They used tris-(4-aminophenyl)amine (TPA) and perylenedianhydride (PDI) as raw materials to synthesize a CMP material with good conductivity, called TPA-PDI. Triphenylamine and its derivatives are well-recognized hole-transport agents. When coupled with electron acceptor building blocks, they can produce a CMP system with inherent conductivity. The special donor-acceptor pair and conjugation of TPA-PDI facilitate the convenient transfer of charges across the CMP structure, which makes this material promising for application in electrocatalysis. They used TPA-PDI to synthesize two UMNPs (Au and Co), called Au@TPA-PDI and Co@TPA-PDI, respectively. Au@TPA-PDI can efficiently catalyze the reduction of nitroaromatics to aminoaromatics. More interestingly, both TPA-PDI and Co@TPA-PDI exhibit considerable ORR catalytic activity without carbonization. Recently, Ag UMNPs@CMPs, Pd UMNPs@CMPs and others have also been reported. 26,90 HCPs are a class of microporous polymers that are prepared by extensive crosslinking of linear or lightly crosslinked precursor polymers. They have high surface area, high porosity and high stability, which make them the candidate materials to support UMNPs. 91 The heteroatom, organic functional groups and hyper crosslinked tubes in HCPs can prevent UMNPs aggregation. 92 Dong et al. prepared thiol-containing HCPs using ferrocenecarboxaldehyde and melamine as building blocks, which were loaded with Pd UMNPs (2.89 nm). 93 The resulting Pd UMNPs@HCP composites showed excellent activity and stability in the catalytic reduction of nitroarenes. Tan et al. reported a series of Au UMNPs (range of 1.7-5.1 nm) supported on a HCP ( Fig. 12b). 22 By simply varying the mass ratio of HAuCl 4 $4H 2 O and the HCP, the Au UMNPs with different sizes can be obtained. These composite materials exhibit high catalytic activity and recyclability toward catalytic reduction of 4-nitrophenol.
Wang et al. presented TB-POP containing triphenylamine and 2,6-bis(1,2,3-triazol-4-yl)pyridyl units. 31 Adding TB-POP into CH 2 Cl 2 solution to adsorb palladium acetate followed by reduction in a stream of H 2 /N 2 at 200 C provided a composite material with Pd UMNPs (1.5 AE 0.6 nm)/TB-POP. Pd UMNPs uniformly dispersed over TB-POP exhibit remarkable catalytic activity and high selectivity in the dehydrogenation of aqueous formic acid solution. Dong et al. fabricated highly dispersed Pd UMNPs (1.4-2.8 nm) loaded on a stable triazinyl-pentaerythritol porous organic polymer (TP-POP) which was synthesized through a facile polycondensation approach between the cyanuric chloride and pentaerythritol (Fig. 12c). 32 The threedimensional structure, abundant triazinyl groups and abundant pores control the growth of Pd UMNPs and prevent their agglomeration during the catalytic process. The as-prepared Pd UMNPs@TP-POP catalyst showed excellent catalytic activity and stability in the reduction of 4-nitrophenol and transfer hydrogenation of aromatic aldehydes under mild conditions.

Synthesis of UMNPs supported by metal oxide/sulfides
Metal oxides/suldes (MX n , X ¼ O or S) are also proved to be efficient support materials for UMNPs. Vacancies or defects are  usually introduced into MX n to promote the interactions between the support and metal atoms or particles. 94 Such strong binding interactions help UMNPs anchor in MX n tightly, stabilizing UMNPs and preventing their aggregation. In addition, oxygen vacancies in metal oxides have oxygen storage capacity, which can play an important role in promoting oxygen involved reactions, such as oxygen evolution, oxygen reduction and CO oxidation. [95][96][97] Moreover, many MX n , such as CdS and TiO 2 , can not only serve as supports for UMNPs, but also have catalytic activity by themselves, synergistically acting as cocatalysts.
Zhang et al. developed a way of in situ embedding Pt UMNPs (1-2 nm) into a nanorod-shaped cerium dioxide (CeO 2 ) through a redox reaction that occurs at the solid solution interface (Fig. 13). 98 CeO 2 has extremely high oxygen storage performance and strong interactions with metals; therefore, is oen used as a support for MNPs. Pt-CeO 2 shows superior thermal stability and durability because of the strong Pt-O bonds formed between Pt UMNPs and O atoms in CeO 2 . The enhanced interface between Pt and CeO 2 is also responsible for the excellent catalytic performance of the hybrid catalyst towards the hydrogenation of nitrophenol. Jiang et al. used Pt ion-doped Ce-MOFs to create Pt UMNPs (<2 nm) supported on CeO 2 particles (s-Pt/CeO 2 ) through ultrafast laser induced reduction. 34 In this process, the MOF crystals absorb laser photons and generate high pressure and high temperature, causing the pyrolysis of organic liners and reduction of metal ions. Nanocrystalline CeO 2 particles with abundant defects on their surface were in situ precipitated, which serve as anchoring sites for Pt UMNPs. Oxygen vacancies in the CeO 2 support oen play a role in promoting CO oxidation. As expected, the s-Pt/CeO 2 can quickly convert CO to CO 2 with a conversion efficiency up to 100%.
The porous structures of supporting materials can increase the reaction rate by accelerating the mass transfer process of the catalytic reactions and improve the stability of UMNPs as well. Various metal oxides are featured as porous structures, which serve as ideal supports for UMNPs. [99][100][101] Zhan et al. supported Pt UMNPs ($3.2 nm) on mesoporous TiO 2 and used them to catalyze the ORR. 102 The strong metal support interaction between Pt UMNPs and porous TiO 2 enhanced the stability of the catalyst, together with the spatial restriction and the antirestriction provided by mesoporous TiO 2 . The catalyst exhibited a much higher stability than the commercial Pt/C aer 10 104 In addition to improving the stability of catalysts, mesoporous Co 3 O 4 provided a high adsorbed reactant concentration due to its porous structure and high pore volume.
Loading UMNPs on some photocatalytic catalysts, such as TiO 2 , is also a good approach for improving specic photocatalytic activity. In many cases, the low photocatalytic     (Fig. 14). 42 The obtained composite lm was used for photocatalytic reduction of CO 2 , which showed extremely high reduction efficiency and CH 4 selectivity. The work of Lin et al. shows the superiority of CdS loaded with Pt UMNPs in the photocatalytic hydrogen evolution reaction. 35 They reduced Pt 4+ into Pt UMNPs (1.75 nm) through ultrasonic radiation and deposited them on CdS nanorods. When the Pt loading was 0.5 wt%, the highest H 2 release efficiency (24.15 mmol h À1 g À1 ) was achieved. The use of Pt UMNPs as co-catalysts on the surface of CdS can effectively promote the separation of photo-generated charges on CdS, offer a low activation potential, and provide active centers for H 2 generation to enhance photocatalytic H 2 evolution.

Synthesis of UMNPs supported by silica
Silica is one of the most abundant components in earth and possesses excellent chemical and thermal stability. It can be considered as an ideal candidate for catalyst supports because of its stability. Silica has a porous structure, especially for ordered mesoporous silica, which can disperse and isolate UMNPs to keep UMNPs stable during catalysis. There are also abundant silanol groups on the silica surface, which makes silica easy to be modied for anchoring UMNPs. 105 Thus, silica has been widely used as supports for heterogeneous catalysts, especially in oxidation or reduction reactions driven by thermal energy such as semihydrogenation, CO oxidation, CH 4 oxidation and so on. [106][107][108] Ding et al. synthesized a series of bimetallic UMNPs (1-3 nm) by decomposing and reducing metal salts adsorbed on silica (Fig. 15). 106 These bimetallic UMNPs exhibit well-dened stoichiometry and intimacy between constituent metals and show excellent catalytic performance in semi-hydrogenation of alkynes. Fedorov et al. reported a highly efficient and selective catalyst for the semi-hydrogenation of alkynes, which is obtained by loading Cu UMNPs (2.0 AE 0.6 nm) on passivated silica. 109 The competing passivation of silanol sites by Me 3 Si groups and low Cu precursor loading limit the density and size of Cu UMNPs on passivated silica. The size of Cu UMNPs and the IMes (1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene) anchored on the Cu surface play important roles in improving the selectivity for semihydrogenation.
Similar to mesoporous carbon, mesoporous silica can provide a suitable conned environment for UMNPs. Yang et al. reported a novel catalyst obtained by supporting Pd UMNPs (1.1 nm) on functional mesoporous silica nanoparticles. 110 Not only the walls of silica can prevent the aggregation of UMNPs, but also the unevenly distributed functional groups in porous silica can maintain the UMNPs in the same channel dispersed. Besides, the isolated silanol groups may interact with phenol through hydrogen bonding and facilitate the phenol hydrogenation process. Kao et al. synthesized Ni UMNPs (2.7 nm) by using 2D hexagonal channel mesoporous silica (SBA-15C) and 3D cage-type mesoporous silica (SBA-16C) as supports. 111 Such a composite catalyst exhibits high catalytic activity and stability in the hydrogenation of nitroarenes to aminoarenes.
Even though mesoporous silica works as an efficient and stable supporting material for UMNPs, most UMNPs supported by mesoporous silica are trapped by pores near the surface rather than deeper pores inside, which may reduce the available surface area and may block the pores. As a unique type of silica, brous silica nanosphere (KCC-1) can provide a much higher surface area for UMNPs due to the brous morphology rather than pore structures. 112,113 KCC-1 will not be affected by the blocking of pores and can provide faster mass transfer. Therefore, compared with mesoporous silica, KCC-1 is superior in various applications, including catalysis. Basset et al. used 3aminopropyltriethoxysilane modied KCC-1 (KCC-1-NH 2 ) as a support and introduced Au ions (Fig. 16). 107 Then they obtained Au nanoparticles of different sizes through different reduction methods. Among them, Au UMNPs (1-2 nm) were obtained using NaBH 4 as a reducing agent. It shows excellent catalytic activity for CO oxidation. The brous structure prevents the migration and aggregation of Au UMNPs, and also it provides a huge contact area with CO.

Conclusion and perspectives
In summary, controlling the synthesis of UMNPs through the use of solid supports can control the size of the UMNPs and help the UMNPs remain well-dispersed. During the catalytic process, the supports and the UMNPs can synergistically determine the catalytic activity. Therefore, synthesizing UMNPs in situ in the presence of a support and controlling the nucleation and growth of UMNPs is a promising method for the controlled synthesis and application of UMNPs in catalysis. The structure and properties of a support play an important role in the growth and application of UMNPs. Many organic porous materials, such as COFs, MOFs, OMCs, and CMPs, can be rationally designed at the atomic level, and their pore size and shape can be precisely controlled, so as to control the size and morphology of UMNPs. The ordered porous support structure is conducive to the narrow size distribution of UMNPs. Carbon materials such as porous carbon, hollow carbon, and graphene have excellent chemical and thermal stability. At the same time, because carbon materials are cheap and easy to obtain, they are economical support materials. Carbon materials doped with heteroatoms or modied with functional groups can provide a large number of nucleation and anchoring sites for UMNPs. The introduction of UMNPs into the vacancies or defects of metal oxides/suldes can rmly bind UMNPs to the supports through strong coupling. In addition, UMNPs can be used as cocatalysts to enhance the photocatalytic activity of metal oxides/ suldes. The controlled growth of UMNPs mediated by solid supports has been summarized in Table 1, including different types of support materials, UMNPs with different types and sizes, and variable applications with catalytic activities.
This review summarizes the control of the growth of UMNPs with solid supports and their applications in the eld of catalysis. Through the analysis of the reported examples, we believe that the following conditions play crucial roles in controlling the growth of UMNPs, while still facing some problems and challenges: (1) The design of supports: porous supports can provide connement for the growth of UMNPs and prevent the accumulation of UMNPs through site isolation. At the same time, the high specic surface area provides a large number of available active sites and anchoring sites. However, in many cases, UMNPs grow outside of the pores of the porous support, which causes the pores to be blocked. Many active sites in the pores cannot participate in the catalytic process, which affects the mass transfer of the catalytic process. Through a proper design, metal precursors can be preferentially loaded inside the pores, which can effectively prevent the growth of UMNPs on the surface of supports. The hierarchical design of micropores, mesopores, and macropores can incorporate the micropores and smaller mesopores, which may help in efficient transfer mass. It is also necessary to consider the interactions between supports and UMNPs to facilitate catalytic reactions while avoiding catalyst poisoning.
(2) Synergistic catalysis provided by UMNPs and supports: some supports can adsorb reaction substrates, which increases the local concentration of reaction substrates near UMNPs and facilitates the catalytic reactions. Bai et al. prepared covalent triazine framework (CTF) nanosheets coated with Ag particles to catalyze CO 2 conversion. 171 CTFs have no obvious catalytic activity for CO 2 conversion, but they have high CO 2 capture ability. The coated Ag particles can catalyze the coupling of CO 2 and phenylacetylene to produce 3-phenylpropiolic acid. The hybrid materials of Ag particles and CTFs can act synergistically to catalyze the reaction. Through the rational design of UMNPs and supports, the catalytic activity of the catalyst can be improved effectively.
(3) Shape and diameter of UMNPs: precise control of the shape and diameter of UMNPs is still challenging, which is critical to improving their performance in heterogeneous catalysis. The spatial connement effect is more inclined to macroscopic design, which can limit the growth of nanoparticles and control the size and shape of nanoparticles to a certain extent. It is still hard to control UMNPs with a size less than 3 nm by using most of the inorganic supports, while electronic effects have a remarkable inuence over the size of UMNPs. The size of UMNPs can be ne-tuned by adjusting the concentration of metal ions and the ratio of heteroatoms, functional groups or defects. The development of new supports that can combine the electronic effects and characteristic spatial shapes may be the future of realizing the precise control of UMNPs, especially for the shape control.
(4) Stability: high stability of the UMNPs and carriers, even under harsh catalytic conditions, is highly desired. Improving the stability of the carrier and UMNPs can increase the number of cycles of a catalyst, which is conducive to reducing the cost of the catalyst and enabling UMNPs for practical applications. Long-term usage or harsh conditions (such as high temperature or corrosive medium) may ruin the structure of solid supports and cause the catalyst to be deactivated. It calls for the development of more stable or even self-repairable supports.
(5) Nucleation and growth mechanism: there have been some studies on the nucleation and growth mechanism of UMNPs on the carrier interface, but the mechanism of nucleation and growth is still unclear and in-depth exploration is needed, which can provide design principles of solid supports and efficient synthetic routes to MNPs.
Although great progress has been achieved, controlling the nucleation and growth of UMNPs through the use of solid supports still faces many challenges. With continuous in-depth research, efficient preparation of UMNPs with well-controlled uniform size and high stability is anticipated, which will enable wide-spread practical applications of UMNPs in the eld of catalysis and beyond.

Conflicts of interest
There are no conicts to declare.