single atom catalysts by atomic diffusion strategy · 2021. 2. 23. · nano res. 1 template for...

ISSN 1998-0124 CN 11-5974/O4 2 https://doi.org/10.1007/s12227-021-3412-9

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Single atom catalysts by atomic diffusion strategy

Min Zhang, Hai-Gang Lu (), and Si-Dian Li ()

Nano Res., Just Accepted Manuscript • https://doi.org/10.1007/s12227-021-3412-9 http://www.thenanoresearch.com on Feb. 22, 2021 © Tsinghua University Press 2021 Just Accepted

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Address correspondence to First A. Firstauthor, email1; Third C. Thirdauthor, email2

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Template for Preparation of Manuscripts for Nano

Research

TABLE OF CONTENTS (TOC)

Single atom catalysts by atomic

diffusion strategy

Lihong Lin, Zhuo Chen,* Wenxing

Chen,*

Energy & Catalysis Center, School

of Materials Science and

Engineering, Beijing Institute of

Technology, China

The atomic diffusion strategy, which can be divided into gaseous diffusion, solid diffusion and liquid diffusion, is considered as an effective method to prepare a series of single atom catalysts.

Provide the authors’ webside if possible.

Wenxing Chen, https://www.x-mol.com/groups/wxchen

single atomic catalyst

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

ISSN 1998-0124 CN 11-5974/O4

2 https://doi.org/(automatically inserted by the publisher)

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Single atom catalysts by atomic diffusion strategy

Lihong Lin1, Zhuo Chen1 (), Wenxing Chen1 ()

1 Energy & Catalysis Center, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China

© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher)

ABSTRACT The depletion of energy and increasing environmental pressure have become one of the main challenges in the world today. Synthetic high-efficiency catalysts bring hope for efficient conversion of energy and effective treatment of pollutants, especially, single-atom catalysts (SACs) are promising candidates. Herein, we comprehensively summarizes the atomic diffusion strategy, which is considered as an effective method to prepare a series of SACs. According to the different diffusion forms of the precursors, we review the synthesis pathways of SACs from three aspects: gas diffusion, solid diffusion and liquid diffusion. The gaseous diffusion method mainly discusses atomic layer deposition (ALD) and chemical vapor deposition (CVD), both of which carry out gas phase mass transfer at high temperatures. The solid-state diffusion method can be divided into nanoparticle transformation into single atoms and solid atom migration. Liquid diffusion mainly describes the electrochemical method and the molten salt method. We hope this review can trigger the rational design of SACs.

KEYWORDS single atomic catalyst, gas diffusion, solid diffusion, liquid diffusion

1 Introduction Catalysts are widely used in the chemical industry and environmental remediation [1]. With the increasing depletion of non-renewable resources such as coal, petroleum, and natural gas, and the increasing requirements of people on the living environment, the development of efficient catalysts is particularly important. Metal nanocatalysts exhibit different activity, stability and selectivity from traditional catalysts due to their volume effect, surface effect and quantum size effect, which greatly promotes the development of nanoscience and industrial applications. When the size of metal nanoparticles is continuously reduced into clusters, sub-nano clusters, or even dispersed into single atoms, the utilization rate of metal atoms will continue to increase, especially after shrinking to single atom, the metal utilization efficiency can reach 100% [2], and the active center gradually evolved from multiple to one [3], which may improve the selectivity [4, 5] and atomic economy of reactants. In addition, as the particle size continues to decrease, the surface energy continues to increase, exposing more unsaturated metal coordination sites [6], which can lead to enhanced chemical reaction activity between the catalyst and the adsorbate [7-9]. Therefore, the effective synthesis and

application of single-atom catalysts or atomic-level dispersed metal catalysts has become a very important research direction in the field of catalysis and materials research in recent years.

After Zhang's group prepared the Pt1/FeOx catalyst by the co-precipitation method [10], single-atom catalysis has aroused great interest among researchers. The single-atom catalyst has a well-defined active center and high metal dispersibility, and it is a new type of catalyst. Unlike the metal atoms in nanocatalysts which are combined through metal-metal bonds, the metal atoms in single-atom catalysts are combined through interaction with the support or charge transfer between the supports, and are anchored to the support in an isolated form. And there is a space barrier between the metal atoms. Due to this unique electronic structure and highly dispersed properties, relevant researchers have developed various methods for the synthesis of single-atom catalysts in recent year, such as mass soft landing method [11], impregnation method [12, 13], electrochemical method [14-17], atomic deposition method (ALD) [18-20], chemical deposition method (CVD) [21-25], pyrolysis method [26-28], freeze-drying method [29] etc.. They loaded the precursors of different formation on the substrate to prepare stable single-atom catalysts, and applied

Address correspondence to [email protected]; [email protected]

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them in different chemical reactions to study their catalytic performance. After several years of exploration, the method of synthesizing single-atom catalysts has developed by leaps and bounds. Researchers have gradually manipulated and arranged atoms in a controlled manner to reduce the amount of metal resources used and the cost of catalysts.

Atoms vibrating at the equilibrium position of the crystal lattice leave their original positions due to higher energy, which results in the phenomenon of atom migration. In this review, we comprehensively summarize the preparation methods of single-atom catalysts by atomic diffusion strategy. When preparing SACs, the atomic diffusion strategy can be divided into gaseous diffusion method, solid diffusion method and liquid diffusion method according to the different diffusion forms of the precursor (as shown in Figure 1). Gaseous diffusion methods include atomic layer deposition (ALD) and chemical vapor deposition (CVD), both of which perform gas phase mass transfer at high temperatures, and the main difference between the two is that the ALD has the characteristics of surface self-limitation and the regeneration of active sites of the carrier [30, 31]. The solid-state diffusion method can be divided into nanoparticle transformation into single atoms and solid atoms migration. The former is the precursor not supported on the carrier, and the latter is the nanoparticle supported on the carrier. Liquid diffusion mainly describes electrochemical method and molten salt method. The former mainly forms an electric field by applying a potential to electroplate the metal ions in the solution onto the electrode. The latter is at a certain temperature, the precursor, co-solvent and carrier form molten salt to derive anion and cation fields to assist the flow of metal ions. According to the different diffusion forms of the precursor, a suitable synthesis strategy is selected to prepare SACs.

Figure 1 Schematic illustration of atomic diffusion strategy for single-atom

catalyst.

A very critical issue when preparing single atoms is how to

stabilize the captured single atoms on the carrier. Surface energy of particles will increase with the particle size decreases, which leading to particle stability be reduced. In order to reduce the surface energy, single atoms are prone to agglomeration or migration, resulting in more stable clusters [32] or particles with larger diameters. In the synthesis process, by selecting the appropriate carrier, the interaction between the metal and the carrier can be improved or the charge transfer between the two can be enhanced, so the single atom can be stabilized at a certain extent. The following types of carriers are usually selected: (1) Defect-rich carriers [33-35], the existence of defects will increase the unsaturated coordination sites of the carrier and improve the charge transfer ability of the carrier, and stabilize it on the carrier while capturing the single atom. (2) Carriers doped with coordinating atoms, introducing coordinating atoms containing lone pairs of electrons [36-39] (such as N, P, S) on the carrier can enhance the interaction between the metal and the carrier and prevent the migration and agglomeration of single atoms. (3) Graphene with containing oxygen functional groups [40, 41] or metal oxides [42-45], which form a strong chemical bond with a single metal atom, so as to achieve the purpose of stabilizing a single metal atom. (4) Porous substances, single metal atoms are encapsulated in the pores of porous substances (such as metal-organic framework (MOF) [46-49], zeolite [50, 51], Metal-organic polyhedra (MOPs) [52]), and their migration and agglomeration are effectively inhibited due to spatial isolation. In addition, there are some carriers that can promote the proximity effect of the catalytic reaction, directly participate in the activation of the substrate, and then increase the catalytic activity [53, 54]. Therefore, it is very important to select a suitable carrier to synthesize SACs.

In this review, we introduce the ALD and CVD in the gaseous diffusion method, which precisely control the growth of single atoms on the carrier in a "bottom-up" manner. At the same time, we use representative research results of predecessors to explain the synthesis process of the gas diffusion method, the synthesis principle and the catalytic performance of the catalyst. And then we introduced the migration of solid-state atoms and the transformation of nanoparticles into single atoms in the solid-state diffusion method, which is a "top-down" synthesis strategy. We emphatically discussed how the "large size" precursor can be transformed into a "small size" single atom under the synthesis conditions, and at the same time explained the catalytic performance of the resulting single atom catalyst. Then, we explain the electrochemical method and the melten salt method in the liquid diffusion method, and introduce how the metal ions are captured by the carrier under the electric field or the anion and cation field to generate the metal single atom catalyst. At the end of the review, we discussed the challenges faced by single-atom catalysts in the synthesis process.

2 Gas diffusion strategy 2.1 Atomic layer deposition（ALD） Atomic layer deposition (ALD) can deposit atoms in the form of a monoatomic film on a substrate in a controlled manner to prepare uniform and conformal thin films [55], which is a the coupling process of precursor diffusion, adsorption and reaction. It is also suitable for substrates with height, depth and width [56], but the


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premise is that the precursor can be chemically adsorbed by the surface of the substrate.

Fig 2 Preparation of single atomic catalyst by ALD method. a) Preparation of Pt/GNS catalyst synthesis. b) 50, 100 and 150 ALD cycles Pt/GNS HAADF-STEM images and corresponding magnified images. ( reproduced with permission from Ref. [20], © Springer Nature 2013). C) Schematic illustration of ALD synthesis of Pd1/graphene catalysts. d) schematic models of Pd-Hfac/graphene (left) and Pd single atom on graphene( right). carbon, oxygen, palladium, fluorine and hydrogen atoms are represented by gray, red, dark cyan, light blue and white pellets, respectively. e) Schematic illustration of selective hydrogenation of 1,3-butadiene on Pd1/graphene catalyst and Pd NPs catalyst. ( reproduced with permission from Ref. [57], © American Chemical Society 2015). f) Schematic illustration of Pd1/C3N4 catalyst synthesized via ALD. g) Aberration-corrected HAADF-STEM images of Pd1/C3N4 catalyst. Highlight the Pd single atoms with white circles. h) a plots of ethylene selectivity vs acetylene conversion of acetylene hydrogenation reaction over Pd1/C3N4 catalyst. ( reproduced with permission from Ref. [18], © Tsinghua University Press and Springer-Verlag GmbH Germany 2017). i) Schematic illustration of dimeric Pt2/graphene catalysts synthesized by Pt ALD on pristine graphene. j) AC-HAADF-STEM images of dimeric Pt2/graphene. (reproduced with permission from Ref. [19], © Springer Nature 2017).

Recently, Sun et al. [20] used the ALD method to deposit Pt on graphene with containing oxygen functional groups to prepare a single-atom catalyst. They oxidized, thermally exfoliated and chemically reduced the graphene nanosheets to produce graphene nanosheets with containing oxygen functional groups, and placed them directly in heating stage in the reactor for the ALD process (Figure 2a). The MeCpPtMe3 was first passed into the reaction chamber and chemically adsorbed on the oxygen-containing functional group sites of graphene at 250 ℃ to form MeCpPtMe2-graphene and by-products. Then, the excess precursors and by-products were blown into the N2 to form a Pt single atomic layer. Then the second precursor was passed into the chamber to form new oxygen-containing functional groups on the carrier. Finally, a complete ALD cycle was completed after blowing through the N2. ALD method relied on continuous saturated surface reactions, which had the characteristics of

self-limitation [58], thus forming a monolayer at each adsorption. Pt monatomic layer was formed by two continuous self-saturated surface reactions during this ALD process. The first self-limiting reaction was the dipole-dipole interaction between MeCpPtMe3 and the substrate surface. The substrate adsorbed MeCpPtMe3 to form a monolayer (MeCpPtMe2-graphene). In the second self-limiting reaction, the added O2 was adsorbed on the carrier, and the surface was restored to the original activated group state, providing active adsorption sites for the next cycle [31, 59]. Sun et al. performed 50, 100, and 150 ALD cycles. They found that the Pt loading under the three cycles were 1.52 %, 2.67 %, 10.5 %, and the average size of Pt particles was 0.5 nm, 1-2 nm, and 2-4 nm, respectively. After multiple cycles, Pt single atoms, sub-nanoclusters and Pt nanoparticles can be observed on the surface of the substrate at the same time (Figure 2b). The result showed that Pt nanoparticles were constantly growing with the


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emergence of single atoms during the cycle [60, 61]. This may be because the number of cycles increased, and redundant nucleation sites are generated, and Pt atoms no longer grew in a single layer.

Yan et al. [57] also used the ALD method to synthesize a single-atom Pd1/graphene catalyst (Figure 2c and 2d). Before ALD, they first oxidized graphene to establish anchor points, and then selected anchor points by thermal deoxygenation, so that the oxygen-containing functional groups remaining on the graphene were mainly phenolic oxygen. At 150°C, gaseous Pd(hfac)2 was used as the first precursor, and phenolic oxygen was used as the active center, so that Pd(hfac)2 diffused on the graphene was fixed on it to form -O-Pd( hfac) and by-products. Then, formalin was introduced to remove excess ligands on the surface of Pd atoms, leaving single atomic layer of Pd on the surface of graphene. They found that the chemical state of Pd single atom on graphene mainly existed in the form of +2 valence, and Pd nanoparticles existed in zero valence, which was consistent with the later research by Huang et al. [18]. In addition, they found that the Pd1/graphene catalyst had a very high selectivity for butene. This may be due to the single π adsorption mode of 1, 3-butadiene and the adsorption of 1, 3-butadiene which could cause the separated Pd atoms to have the space effect, so the secondary hydrogenation was effectively suppressed (Figure 2e).

Unlike the former, which used the Pd-O bond formed by the combination of Pd single atom and a phenoxy functional group to stably capture the Pd single atom. Huang et al. [18] used the charge transfer between the single Pd atom and graphite carbon nitride (g-C3N4) to stabilize single Pd atom. The synthesis process was shown in Figure 2f and 2g, They used Pd(hfac)2 as the first precursor, N2 as the purge gas, and formaldehyde as the second precursor, and performed ALD at 150°C to fix the single Pd atom in six-fold cavities. Predecessors used density functional theory to calculate the binding energy of Pd atoms and six-fold cavities, and found that the binding force between them was very strong [62], which made the metal atoms more inclined to combine with the six-fold cavity, and formed Pd single atoms instead of Pd NPs on g-C3N4. Then they tested the catalytic performance of Pt particles synthesized by different methods (as shown in Figure 2h). When the conversion rate of these three catalysts (Pd1/C3N4, Pd/C3N4-NP(ALD), Pd/C3N4-NP(WI))was 99 %, the selectivity of ethylene was 83 %, 35 %, and 22 %, respectively. Pd1/C3N4 selectivity changed steadily, while Pd/C3N4-NP (ALD) and Pd/C3N4-NP (WI) rapidly dropped below 80 % after the conversion rate was greater than 30 %. Under high conversion rate, Pd single atom had higher ethylene selectivity [63]. The single-atom catalysts prepared by ALD show good catalytic performance in hydrogenation reactions. Does it also promote hydrogen evolution reactions? Cao et al. [64] studied this and analyzed the effect of single-atom Co loaded on phosphating treated g-C3N4 for the hydrogen evolution reaction (HER). They used ALD and Co(Cp)2 as a gaseous precursor to prepare a Co single-atom catalyst with a Co1-N4 structure. They found that the two H* forming H2 were adsorbed on the Co and N positions, respectively. The Co on the Co1-N4 structure acted as the active center, and the N site assisted the active center to form a hydride intermediate, thereby promoting the H-H bond bonded and improving the efficiency of hydrogen evolution. It revealed that the coordination atoms around the metal atoms can affect the catalytic performance of the catalyst [65, 66]. As shown in Table 1, most of the current SACs were coordinated in

a metal-nonmetal way (M-NxOxCx), and used this as a catalytic active site to improve the catalytic performance of the catalyst.

In 2014, according to the unique self-limiting property of ALD and the choice of deposition temperature, Lu et al. [67] selectively deposited another layer of nanoparticles on Pt nanoparticles to prepare bimetallic nanoparticles. Unlike the former, Yan et al. [19] used the ALD method to precisely control the generation of single atoms in a controlled manner. On this basis, metal atoms were selectively redeposited to prepare metal dimers (Figure 2I). They prepared Pt1/Graphene by controlling the production of isolated anchor sites on the original graphene. In the first cycle, MeCpPtMe3 and molecular O2 were alternately exposed at 250°C to form suitable nucleation sites to prepare Pt1/Graphene. In the second cycle, changed the reaction temperature to 150°C, selectively deposited MeCpPtMe3 on the Pt atoms of Pt1/Graphene, and used a strong oxidant O3 to remove excess ligands to prepare Pt dimers (Figure 2j). They conducted nine independent experiments to prove whether metal atoms are selectively redeposited on Pt1/Graphene. The results show that the ratio of Pt load on Pt2/Graphene and Pt1/Graphene is two. Although the products of the two were different, they were prepared by using the characteristics of ALD method that could precisely control the nucleation site. It can be seen that the precise controllability of nucleation sites is a major advantage of the ALD method for preparing catalysts.

According to the above description, the self-limiting surface reaction and the number of cycles of ALD can make the metal atoms supported on the carrier in an isolated form. The newly formed bond between the metal atoms and the oxygen-containing function on the surface of the carrier, or the spatial barrier of the six-fold cavity, enables these isolated metal atoms were anchored on the carrier, while hindering their migration and agglomeration to a certain extent. The temperature will affect the uniformity and adhesion of metal atoms on the carrier. If the temperature is too low, the reaction may not proceed normally due to insufficient energy. Too high temperature will not only cause agglomeration of attached atoms, but may also cause the precursor to decompose and cause side reactions. Although the ALD method can successfully prepare stable single-atom catalysts, the cost is relatively high and the load is low, and as the number of cycle increases, sub-nano clusters, clusters or nanoparticles will gradually appear on the carrier. How to increase the loading capacity of metal atoms while maintaining the dispersion of metal atoms needs further exploration. 2.2 Chemical vapor deposition（CVD） CVD is a vapor growth technology that can convert one or more raw materials into volatile substances after heat treatment, which reaches the surface of the carrier with the help of carrier gas, is captured by defects on the carrier, and then undergoes a chemical reaction on the surface of the carrier. When the temperature in the reactor is heated to the required decomposition temperature of the compound, isolated metal sites can be formed on the carrier. CVD is a "top-down" preparation technology that can be used to prepare 2D materials [68, 69], nanoparticles [70], and single-atom catalysts. The key point is whether the precursor can chemically react at the gas phase or gas-solid interface and the reaction conditions are controlled.


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Fig 3 Preparation of single atomic catalyst by CVD method. a) Synthetic apparatus and proposed reaction mechanism of Cu-SAs/N-C. b) TEM image of ZIF-8 (b1) and magnified HAADF-STEM image of Cu-SAs/N-C (b3) in the absence of NH3 , TEM image of ZIF-8 (b2) and magnified HAADF-STEM image of Cu-SAs/N-C(b4) in the presence of NH3. c) K3-weighted χ(k) function of EXAFS spectra of the Cu-SAs/N-C catalyst and reference samples. d) FT-EXAFS fitting curves and Cu-N4 local structures of the Cu-SAs/N-C catalyst. ( reproduced with permission from Ref. [71], © Nature Publishing Group 2018). e) Synthetic apparatus and Proposed reaction mechanism of Cu ISAS/N-C. f) The single atoms catalyst was prepared by changing the carrier and the metal precursor. ( reproduced with permission from Ref. [72], © Nature Publishing Group 2019). g) Synthetic scheme of the xCVD/Fe N-C-kat catalysts with different Fe content. h) The XANES spectrum of the 0.17 CVD/Fe-N-C-kat catalyst. i) The EXAFS spectrum of the 0.17 CVD/Fe-N-C-kat catalyst. (reproduced with permission from Ref. [73], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2020).

Qu et al. [71] used the CVD method, assisted by ammonia gas, to synthesize Cu single-atom catalysts from bulk metal Cu (Figure 3a). The Cu foam and the zeolite imidazole skeleton-8 (ZIF-8) were placed on a porcelain boat, and Ar gas was introduced to make ZIF-8 undergo pyrolysis at 1173 K, and Zn atoms were volatilized. Then, ZIF-8 became a nitrogen-rich porous carbon support with empty zinc nodes and abundant defect sites. The gas was switched to ammonia, and the ammonia molecules pulled out the surface copper atoms on the Cu foam to generate gaseous Cu(NH3)x species. Then, the Cu(NH3)x species flowed with the ammonia gas onto the carrier and was trapped by the defect, forming single Cu atom on the empty zinc node. They used ammonia gas as the medium, dragged Cu atoms out of the bulk Cu and anchored them on the nitrogen-rich carbon carrier, so that the single Cu atoms were loaded on the carrier in the form of 0 to +2 valence (as shown in the figure 3c), forming Cu-SAs catalyst with atomically dispersed Cu-N4 active centers (Figure 3b4, 3d). When the gas was not switched to ammonia, there was no single Cu atom

on the carrier (as shown in figure b3), and the morphology of ZIF-8 remained in the initial state (as shown in figure 3b1). However, after the ammonia gas was introduced, and the morphology of ZIF-8 showed ups and downs at the boundary (Figure 3b2). It embodied that ammonia molecules could coordinate with Cu atoms on the surface of Cu foam through strong Lewis acid-base interactions, so that Cu was separated from the bulk Cu in the form of atoms, and diffused to the surface of the carrier with the gas flow.

Ammonia is corrosive, and has higher requirements on the corrosion resistance of production equipment, which is disadvantageous to production. In order to reduce the corrosiveness of the production process, Yang et al. [72] modified the synthesis method. They did not use ammonia as a carrier fluid, and prepared metal single-atom catalysts directly from bulk metal oxides in N2

atmosphere (Figure 3e). First put the cuprous oxide and defect-rich nitrogen-doped carbon (NC) carrier on the porcelain boat. Then,

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Cu2O was vaporized under 1273K and flowing N2, and the volatile Cu2O flowed with N2 onto the carrier and was trapped and reduced by defects, forming a Cu-SAs catalyst with atomically dispersed Cu-N3 active centers with C vacancies. As shown in Figure 3f, after changing the support to N-doped reduced graphene oxide (N-rGO) and N-doped carbon nanotube N-CNTs, they also successfully synthesized Cu single-atom catalysts. Not only that, changing the precursors to oxides of Mo and Sn, they also successfully synthesized single-atom catalysts with different metal atoms. This method provided a general strategy for preparing metal single-atom catalysts from bulk metal oxides.

Different from the former two, Liu et al. [73] synthesized the SACs catalyst by changing the reaction temperature many times during the CVD process. When performing chemical vapor deposition, the deposition temperature affected the uniformity and chemical composition of the product by affecting mass transfer and interface reactions [74]. As shown in Figure 3g, they put 2-methylimidazolate (2-MeIm) and Fe-ZnO nanosheets into two alumina combustion boats, and heated them in two furnaces with different temperature zones. After heating to 280°C, 2-Melm was

vaporized, and then flowed with argon to Fe-ZnO nanosheets and deposited on it to form Fe-Zn(MeIm)2 intermediates. As the temperature was heated to 350°C, the phase state of the intermediate changed from zif to kat, it was then calcined at 1000°C for in-situ thermal activation to form xCVD/Fe-NC-kat. The most suitable iron content was 0.17( The 0.17 here refers to the iron content in the Fe-ZnO carrier.), which had a well-dispersed atomic Fe site and N coordination (Figure 3h, 3i). Interestingly, they found that the 0.17 CVD/Fe-N-C-kat. Catalyst had the highest ORR activity, which may be due to the increase in FeN4 active site density during in-situ thermal activation at 1000°C and the intrinsic activity associated with shortening the Fe-N bond length was enhanced at this temperature.

Hence, in the CVD process, the local structure formed by the combination of the reaction product and the defect-rich carrier or the carrier doped with coordination atoms containing lone electron pairs can enhance the interaction or charge transfer between the metal and the carrier. To a large extent, these single atoms can be fixed, inhibit the migration ability of single metal atoms, and prevent them from forming larger and more stable nanoparticles.

Table 1 Summary of SACs by gas diffusion.

Synthesis

methods

catalyst Coordination

models

Load

(%)

Reaction

type

Ref.

ALD

Pd1/Graphene Pd-O3C1 0.25 wt Hydrogenation [57]

Pt/Graphene Pt-Cx or Pt-Ox. --- MOR [20]

Pd1/C3N4 Pd-Nx 0.5 wt hydrogenation [18]

Pt1/graphene Pt-O4 --- Hydrolytic

dehydrogenation

[19]

Co1/PCN Co1-N4 1.0 wt HER [64]

Pt/NGNs Pt–Ox or Pt–Cx 0.19 wt HER [60]

Co1/G SACs Co1–O2C4 0.4 ~2.5 wt Hydrogenation [75]

10c-Fe/MWCNTs Fe-Ox 0.36 wt CO oxidation [76]

15c-Fe/SiO2 SACs Fe-Ox -- 1.78 wt CO oxidation [76]

Pt1/CeO2 Pt-Ox 0.22 wt CO oxidation [77]

CNT@SACO --- 2.9 wt --- [78]

CVD

Cu-SAs/N-C. Cu–N4 0.54 wt ORR [71]

Cu ISAS/NC Cu-N3- V 0.45 wt ORR [72]


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xCVD/Fe-N-C-kat FeN4 0.15 at ORR [73]

3 Solid diffusion strategy 3.1 Nanoparticles become single atoms

Catalysts play an important role in the chemical industry and in facilitating catalytic reactions. However, as the reaction progresses, the metal atoms on the catalyst will agglomerate, which will reduce the catalytic activity of the catalyst. By redispersing the aggregated metal particles, it is expected to restore the catalytic activity of the catalyst.

High temperature is beneficial to atomic movement, which may lower the barrier required for bonding. The difference is that although lower temperature will increase the energy barrier required for atom diffusion, it can inhibit the nucleation process of atoms. In addition, High temperature promotes the formation of the metal atom-support bond, and at the same time can enhance the stability of the bond. However, it is also easy to cause agglomeration of metal atoms, sintering occurs, and then particles

with larger diameters are formed. Therefore, the selection of the synthesis temperature is very important, it determines the bonding form of the bond to a certain extent, and has a great influence on the formation of a single metal atom. In this regard, we have listed the reaction temperature for the synthesis of SACs by the solid-state diffusion method of metal atoms, as shown in Table 2. Unlike sintering, the solid-state diffusion method breaks the metal-metal bond between nanoparticles or bulk metals and anchors single metal atom on the carrier in the form of a metal-carrier bond. The solid-state atom migration is when the bond between the metal atoms on the surface of the bulk metal were broken, the metal atoms are separated in the form of single atoms, and then loaded on the carrier. Nanoparticles become single atoms through heat treatment or thermal oxidation process to break the bonds between metal nanoparticles, and then dispersed into single atoms.

Fig 4 Nanoparticles become single atoms. a) Schematic images of single Pt atom and Pt clusters reversible interconversion in H2 and O2 atmospheres. b) Aberration-corrected HAADF-STEM images of the reduced Pt-CHA-2 catalyst at different times after exposure to oxygen atmosphere. c) and d) Time-resolved XANES spectra about gradual reduction of the metal in the oxidation of Pt-CHA-2 in 4% H2 and gradual oxidation of the metal in reduced Pt-CHA-2 in 20% O2 ,


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respectively. (reproduced with permission from Ref. [79], © American Chemical Society 2016). e) Schematic images of Pt NPs (ceria-free and in the existence of polyhedral ceria or rods ceria) sintering. f) HAADFAC-STEM images of Pt/ceria rods which before (above) CO oxidation measurements and after (below) three cycles of CO oxidation. g) a plots of CO conversion vs temperature of CO oxidation reaction over Pt/La-Al2O3 mixed ceria powder.( reproduced with permission from Ref. [80], © American Association for the Advancement of Science 2016).

For example, Moliner et al. [79] encapsulated precious metal Pt nanoparticles in the pores of high-silica zeolite with small pores, and dispersed the Pt nanoparticles into Pt single atoms under oxygen activation. As shown in Figure 4a, firstly, N, N, N-trimethyl-1-adamantane ammonium (TMADA) was used as the organic structure directing agent, and H2PtCl6·6H2O was used as the precursor to prepare a diameter of ~1 nm Pt nanoparticles in a high-silica zeolite with small pores. Then, under an O2 atmosphere and a calcination temperature of 450-650°C, ~1 nm Pt nanoparticles were separated in situ to form stable Pt single atoms. After the single Pt atom was calcined at 80°C, H2 was introduced and the calcination temperature was increased to 150~650°C, the single Pt atom can be reformed into ~1 nm Pt nanoparticles. Under oxidation and reduction atmospheres, single Pt atom and ~1 nm Pt nanoparticles could be converted to each other, forming a reversible cycle. HAADF-STEM images (Figure 4b) showed that the initial Pt nanoparticles gradually disappeared in situ after being exposed to O2. The time-resolved XANES spectrum showed that the white line intensity gradually decreased until the white line intensity of the Pt foil was the same when the temperature gradually increased under the H2 atmosphere (Figure 4c), indicating that the Pt in the oxidation state in Pt-CHA-2 was gradually reduced. On the contrary, when the temperature was gradually increased in an O2 atmosphere, the intensity of the white line gradually increases (Figure 4d), indicating that the reduced Pt species were gradually oxidized. In this method, under the oxidizing or reducing atmosphere, the Pt species was transformed between the reduced state and the oxidized state through the control of different calcination temperatures to complete the mutual transformation between Pt nanoparticles and Pt single atoms.

The former separated nanoparticles into single atoms in situ under a relatively high temperature oxidizing atmosphere. Jones et al. [80] took a different approach, using different processing methods to disperse the nanoparticles of alumina on the ceria. They physically mixed Pt/La-Al2O3 with ceria, then emitted Pt in gaseous form at high temperature, and then used a carrier to capture it, and then prepared anti-sintering and atom-dispersed Pt catalyst. As shown in Figure 4e, at 800°C, the mixed Pt/La-Al2O3 and ceria were aged in flowing air, and the single Pt atom emitted from Pt/La-Al2O3 was captured by the polyhedral ceria and stabilized at Polyhedral ceria surface. Pt/La-Alumina without cerium oxide, Pt nanoparticles sintered quickly. Pt/La-Alumina mixed with cubic ceria could slow down the sintering rate of Pt nanoparticles. The Pt/La-Alumina with ceria rod was aged and subjected to CO oxidation three times, and then observed with HAADFAC-STEM, the image incarnated that the Pt species

remained atomically dispersed (Figure 4f). In the CO oxidation reaction (Figure 4g), 1 wt% Pt/La-Al2O3 mixed with polyhedral cerium had no significant change in reactivity after aging for 1 week, and had obvious anti-sintering ability. 1 wt% Pt/La-Al2O3 mixed with polyhedral cerium and nanorod cerium, which exhibit similar reactivity, while the mixture with cubic cerium had the lowest catalytic activity. Similarly, Lang et al. [81] reported the synthesis strategy of Pt NPs atomically dispersed into Pt single-atom catalysts. They put the nanoparticles loaded on Fe2O3 in oxygen and calcined at a lower temperature. Then it was annealed at 800°C in argon atmosphere, Pt was separated from the host in the form of PtO2, and then captured and reduced by the reducing Fe2O3 to form an atomically dispersed 0.3Pt/Fe2O3-C800 catalyst.

In the process of nanoparticle becoming single atom, the nanoparticle or cluster anchored on the carrier gradually shrinks in size at a certain temperature, and finally shrinks to a single atom. In this process, the sintering and atomization of particles coexist, one of them is dominant at different temperatures, and the two are a competitive relationship, so it is very important to control the reaction temperature.

Yang and his colleagues [82] adopted a "top-down" strategy to convert Ni NP loaded on defect-containing NC carriers into single Ni atoms. First, Ni NPs were loaded onto the surface of the NC carrier (denoted as Ni NPs @ NC) by stirring and centrifugation. Then, under the Ar atmosphere and 1173K, Ni NPs broke the C-C bond on the surface of the carrier and entered the NC carrier through thermal diffusion. At the same time, due to the strong metal-carrier interaction, Ni atoms were continuously split from the Ni NP, and the atoms dispersed On N-rich defects, pores were continuously formed on the surface of the carrier (Figure 5a). The environmental TEM image visually showed how Ni NPs were transformed into Ni single atoms step by step (Figure 5b). When the temperature increaseed from 298 K to 673 K, the average size of Ni NPs increaseed due to agglomeration. However, as the temperature continued to rise, these Ni NPs atomized and eventually disappeared. The results of CO2 SE-Ni SAs@PNC and Ni SAs@NC embodied that, between -0.5 V and -1.2 V, the TOF of SE- @PNC was significantly higher than that of Ni SAs@NC (Figure 5c); As shown in Figure 5d, When the applied potential was between -0.7 V and -1.2 V (vs RHE), the FE of NC and Ni NPs@NC samples was less than 1%. In contrast, the FE of the SE-Ni SAs@PNC catalyst was higher than 80% and even close to 100% at a potential of -0.6 V~ -1.0 V, indicating that Ni single atom could be used as the active center of CO2 electroreduction, this was consistent with the research results of Liu et al.[85].


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Fig 5 Synthesis, structure, and catalytic properties of nanoparticles transformed into single atoms. a) Schematic images about the Ni nanoparticles were transformed into Ni single atoms by pyrolysis. b) Environmental TEM images which Ni NPs is converted into Ni SACs at different temperatures. c) TOF test diagram for SE-Ni SAs@PNC (red histogram) and Ni SAs@N-C (green histogram) in CO2 electroreduction. d) FE diagram for SE-Ni SAs@PNC and reference samples in CO2 electroreduction. (reproduced with permission from Ref. [82], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2018). e) Schematic images and corresponding movie TEM images of a pyrolysis process in which Bi-MOF is transformed into Bi SAs/NC. f) K3-weighted χ(k) function of EXAFS spectra of the Bi SAs/NC catalyst and reference samples. d) FT-EXAFS fitting curves and Bi-N4 local structures of the Bi SAs/NC catalyst. h) Diagram of CO2RR mechanism on Bi SAs/NC catalyst. i) and j) LSV curves and FECO (%) of Bi SAs/NC and reference samples in CO2RR, respectively. (eproduced with permission from Ref. [83], © American Chemical Society 2019). k) Synthesis route (above) and HAADF-STEM images (below) which Pd-nanoparticles@ZIF-8 is converted to Pd single atoms via pyrolysis process. l) Representative environment TEM images which Pd NPs evolved into Pd single atoms by calcining Pd-NPs@ZIF-8. m) High-resolution HAADF-STEM image of Pd-SAs catalyst. Highlight the Pd single atoms with white circles. n) and o) Acetylene conversion and ethylene selectivity of Pd-SAs-900 catalyst at different reaction temperature(80℃~160℃), respectively. (reproduced with permission from Ref. [84], © Springer Nature 2018).

In addition to atomizing nanoparticles under heat treatment,

other substances could also be added to assist the atomization of nanoparticles. For example, Zhang et al. [83] used Bi-MOF as the precursor to pyrolyze Bi-MOF into Bi single-atom catalyst (Bi SAs/NC) with the assistance of NH3. As shown in Figure 5e, under flowing Ar gas, Bi-MOF and dicyandiamide (DCD) were placed in a porcelain boat in sequence. Under Ar gas pressure of 1.45 mbar, the pyrolysis Continue to raise the temperature to 500°C, the Bi NPs on the edge of Bi-MOF-derived nanobundles evaporated, and DCD decomposed to release NH3. Temperature was gradually heated from 23°C to 600°C. Below 450°C, Bi-MOF gradually evolved into Bi NPs. Continue to increase the temperature, with the assistance of NH3, the Bi NPs further atomized, and at the same

time further promoted the doping of N in the C network, and finally formed the Bi SAs/NC catalyst with the structure of Bi-N4. The EXAFS spectrum of R space (Figure 5f) analysis showed that the appearance of a high intensity peak at 1.47Å indicated that Bi SAs/NC had strong Bi-N coordination. According to EXAFS fitting analysis, the local structure of Bi SAs/NC was that one Bi atom coordinates with four N atoms (Figure 5g). In CO2 reduction (as shown in Figure 5h), BiN4/C had a complete CO2RR reaction on the surface. The addition of Bi SAs/NC activated CO2 to form COOH* intermediates. BiN4/C had a lower Gibbs Free energy for the formation of COOH* could reduce the initial potential of CO2

reduction to CO. This was consistent with the mechanism that Ni catalyst with Ni-N4 coordination structure catalyzed the reduction

Nano Res. 10

of CO2 to CO. The formation of *COOH intermediate was released from the surface of the Ni catalyst. In the presence of Ni-N4 structure, the *COOH intermediate could be converted to *CO Species, and finally formed CO [86]. The test results reflected that the total current density of Bi SAs/NC in the test voltage range was higher than that of Bi CS/NC and Bi NPs/NC (Figure 5i); The FE of Bi SAs/NC in the entire test voltage range was significantly higher than the other two catalysts, and its FE could reached 97% at -0.5 V, indicated that its catalytic activity was higher (Figure 5j).

The metal atoms of the supported metal catalyst were dispersed on the carrier in single form, which could maximize its atom utilization rate and reduced the production cost of the catalyst, which was especially important for precious metal catalysts. Wei et al. [84] used pyrolysis to disperse Pd nanoparticles on the NC carrier derived from ZIF-8. As shown in Figure 5k, the Pd-NPs, Zn(NO3)2 solution and 2-methylimidazole solution were mixed to prepare a ZIF-8-coated NPs complex (Pd-NPs@ZIF-8). Then, Pd-NPs@ZIF-8 was pyrolyzed at 900°C in an inert atmosphere. Within 0.5 h of the beginning of heating, Pd-NPs became larger due to sintering. After heating for another 1.5 h, part of the NPs would be atomized and became smaller, and the crystalline Pd nanoparticles would turn into an amorphous state. Continue heating, the remaining nanoparticles gradually atomized and eventually disappeared, and the formed single Pd atom (as shown

in Figure 5m) was supported on the NC carrier (Pd-SAs) derived from ZIF-8. In situ ETEM (Figure 5l) revealed the internal mechanism of the evolution of Pd NPs into Pd SAs. During the process of increasing the calcination temperature from 100°C to 900°C, the average diameter of NPs showed an increasing trend, but the number decreased, indicating that the sintering process was dominant and accompanied by atomization. After raising the temperature to 1000°C, the atomization process dominates. Within 0~36 s of calcination at this temperature, NPs’ agglomeration and atomization were accelerated, and then the size of NPs decreaseed, and finally disappeared completely, forming a local structure single Pd atom of Pd-N4. In the semi-hydrogenation of acetylene (as shown in Figure 5n, 5o), when the reaction temperature was 120°C, the conversion rate of Pd-SAs to acetylene (96.0%) and the selectivity of ethylene (93.4%) were higher than the other four catalysts. At 140°C, when the conversion rate of Pd-NPs/CN to acetylene rose to 100%, the selectivity dropped drastically, only 47.2%. When the conversion rate of Pd-SAs to acetylene reached 100%, the selectivity decreased but did not decrease significantly, indicating that Pd-SAs had better catalytic performance. Feng et al. [42] also came to the same conclusion. They used the synthesized NP-Pd/MPNC and ISA-Pd/MPNC for the semi-hydrogenation of acetylene, and the results showed that the latter has higher ethylene selectivity and stability.

Fig 6 CNT@PNC/Ni SAs synthesis schematic and performance characterization. a) Schematic of CNT@PNC/Ni SAs synthesis via pyrolysis method. b) Schematic diagram of nanoparticle digging and atomization process on CNT. c) In-situ TEM images CNT@NC@Ni NPs at 900℃. d) HAADF images of CNT@NC@Ni NPs and CNT@PNC/Ni SAs, indicating NPs digging effect during calcination. e) and f) The products and the selectively of methane oxidation for CNT@PNC/Ni SAs with different Ni loading(expressed as 3, 4, 5, respectively), reference samples are blank groups, CNT@NC, CNT@NC@Ni NPs(expressed as B1, B2, 1, 2, respectively). The line chart represents the Ni load. (reproduced with permission from Ref. [87], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2019).

The direct use of polymers containing N atoms to construct coordination centers on the surface of the carrier is another effective way to convert nanoparticles into single atoms. In the

conversion process, the nanoparticles loaded on the carrier were gradually lost during the heating process, and finally form single atoms in situ. For example, Zhou et al. [87] used two steps to load


Nano Res. 11

Ni nanoparticles on modified carbon nanotubes and then convert them into Ni single atoms to prepare sintering resistant Ni single-atom catalyst. As shown in Figure 6a and 6b, carbon nanotubes (CNT@PDA for short) firstly were coated with N-containing polymer (polydopamine), and Ni nanoparticles were anchored on CNT@PDA. Subsequent to carbonization treatment under N2 protection and 600°C for one hour, the PDA layer was transformed into an amorphous N-rich carbon layer (CNT@NC for short). Ni NP adapted to the surface pores, sinking continuously from the surface of the amorphous carbon layer, and was fixed in the surface pores of the carbon support. Then, the annealing temperature was continuously increased to 900°C, and the Ni NP was continuously atomized at the original position to form Ni SAs of Ni−N4 architectures (referred to as (CNT@PNC/Ni SAs)) in the hole. In-situ TEM images showed that the size of Ni NP with a diameter of 4.8 nm at 900°C decreaseed with the extension of the annealing time, especially during the 30-36 s period, the particle diameter shrank from 4.6 nm to 1.6 nm. In the following 14 s, Ni NP disappeared and transformed into Ni SAs (Figure 6c, 6d). In the oxidation of methane (as shown in Figure 6e, 6f), the yields of CNT@NC and CNT@NC@Ni NPs were very low, and the selectivity of catalyzing the CH3OH reaction was not high. When catalyzed by single-atom-containing Ni catalyst, and the yield and the selectivity was significantly improved. The catalysts with

different Ni loadings also exhibited different catalytic performance. When the loading was 0.68%, the yield could reached 1.063 μmol·h-1·mg-1

cat, and the selectivity for catalyzing the CH3OH reaction could reached 94.2%.Continue to increase the loading of Ni to reach 1.07 wt%, the yield and the selectivity of the catalytic reaction decreased. 3.2 Solid atom migration To synthesize single-atom catalysts by solid-state atom migration strategy, the choice of carrier is very important. The single metal atom detached from the bulk metal depends on the defect, coordination atom or functional group on the carrier to trap them. At the same time, the stability of a single metal atom on the carrier is poor, and it is necessary to use these characteristics of the carrier to establish a metal-support interaction to stabilize the single atom and inhibit its sintering. Different from the preparation of single atoms from nanoparticles, when solid atoms migrate, the bulk metal gradually disappears, and the lost metal atoms are attached to the carrier in an isolated form, thereby reducing the waste of metal resources. In addition, the direct preparation of single atoms from bulk metals can simplify synthesis routes and reduce production costs.


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Fig 7 To synthesize single-atom catalysts by solid-state atom migration strategy. a) Schematic illustration of conversion of bulk Ni into Ni single atom. b) Water contact angles (CA) and Underwater gas bubble contact angles of H-CPs and Corresponding schematics (Upper parts). CA and Underwater gas bubble contact angles of Ni SAs/CFPs and Corresponding schematics (lower parts). c) Specific current density of H-CPs and others electrocatalysts as a function of potential. d) In CO2

Electroreduction, interfacial charge-transfer resistance of H-CPs and F-CPs. e) Diagram of current density and FE changing with time. ( reproduced with permission from Ref. [88], © Cell Press 2019). f) Schematic illustration of preparation of Pt SAs/DG catalysts by thermal emission strategy. g) HAADF STEM image of DG (Upper left) and M SAs/DG (M=Pt, Pd, Au, Corresponding to the lower lef, upper right, lower right, respectively). h) Charge density difference plots after anchoring Pt single atoms on pristine graphene (left) and graphene oxidation (right). i) Chart of mass Activity of Pt SAs/DG and reference samples in HER. j) The tafel slope of the Pt SAs/DG and reference samples. k) A schematic diagram for the synthesis of Fe SAS/GO catalysts prepared by trapping Fe atoms using dangling bonds on graphene oxidation. (reproduced with permission from Ref. [89], © American Chemical Society 2019). i) k3-weighted χ(k) function of the EXAFS spectra of Fe SAs/GO and reference samples. m) The EXAFS fitting curve and Schematic models for Fe SAs/GO, where the balls in green, grey and red represent Feδ+, carbon, oxygen atoms, respectively. (reproduced with permission from Ref. [90], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2019).

Zhao et al. [88] used the unsaturated carbon vacancies on the carrier to capture and stabilize the Ni NPs that diffused from the bulk Ni to the carrier, and then etched with acid, and single Ni atom was anchored on the carbon vacancy of the carrier. As shown in Figure 7a, they used an air brush to coat the melamine on the surface of the Ni foil, and put it into a porcelain boat after drying. Then, in Ar atmosphere, the temperature in the tube furnace was increased from room temperature to 1000°C at a certain heating rate, and kept for 7 h, the melamine film was gradually transformed into a C3N4 structure. Under the catalysis of Ni, C3N4 gradually transforms into an N-C structure with abundant

unsaturated carbon vacancies. Due to the strong Lewis acid-base interaction between Ni and the N-C layer, Ni diffused from the Ni foil to the carbon vacancies of the N-C layer, and C atoms entered the surface of the Ni foil from the N-C layer. On the surface of the N-C layer, Ni NPs were acted as "seeds" for the growth of carbon-doped nitrogen nanotubes. After cooling at room temperature, the carbon paper peeled off the surface of the Ni foil (F-CPs). Finally, the F-CPs were soaked in acid, and Ni atoms were anchored on the surface of N-CNTs (H-CPs) in an isolated form. As shown in Figure 7b, compared with Ni SAs/CFPs, H-CPs were super-hydrophilic, and their water contact angle (CA) was


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close to 0°, which facilitated the wetting between the electrode and the electrolyte and improves the catalytic performance [91]. Not only that, the CA of bubbles attached to the surface is 148.3 ± 2.6°, indicating that H-CPs were beneficial to the desorption of gas in the aqueous solution. In CO2 electroreduction, H-CPs had better catalytic performance than the catalysts listed in Figure 7c. Figure 7d showed that the interface charge transfer resistance (RCT) of F-CPs was higher than that of H-CPs, indicating that F-CPs was not as capable of electron transfer as H-CPs. In addition, H-CPs were subjected to a stability test at -1.0 V for more than 40 h, and the results showed that neither Faraday efficiency nor current density produced large fluctuations, nor did they experience significant attenuation (Figure 7e).

The diffusion of metal atoms plays an important role in the synthesis of single-atom catalysts. By adding a certain species to assist the diffusion of metal atoms, the carrier can capture single atoms. There is a strong Lewis acid-base interaction between ammonia and metal, which makes solid metal easily converted into volatile metal species and enhances its flow ability. Qu et al. [89] used a thermal emission strategy to prepare single-atom catalysts with the aid of ammonia. As shown in Figure 7f, dicyandiamide (DCD), Pt mesh and graphene oxide (GO) were placed in sequence in the argon flow direction. When the temperature in the device was increased to 1100°C, DCD underwent pyrolysis and ammonia gas was released. Metal cations could be used as Lewis acid-base centers to cause strong coordination between ammonia and Pt atoms [92]. The difference in ammonia charge density could separate Pt atoms from the Pt network to form possible volatile Pt(NH3)x species, which were carried to the GO surface. The oxygen-containing functional groups on the GO surface oxidize Pt0 to Ptδ+ (0 < δ < 4) species [93], which were removed by heat treatment to form defective graphene (DG). The carbon defect on DG captures Ptδ+ (0 < δ < 4) species, forming a Pt SAs/DG catalyst with a local structure of Pt−C4 (Figure 7g2 ). In the absence of DCD synthesis catalyst, GO did not have Pt atoms (Figure 7g1), indicating that DCD cpuld assisted GO to capture metal atoms. The difference graph of charge density (Figure 7h) embodied that compared with the original graphene, the electronic structure of Pt single atom loaded on DG had undergone a greater change, and more charges have been transferred. When the metal precursor was replaced with bulk Pd and Au, DG also successfully captured single metal atoms (as shown in Figures 7g3 and 7g4), indicating that the method was versatile. In the HER reaction, Pt SAs/DG showed more excellent catalytic performance. When the current density was 10 mA·cm−2, the overpotential of Pt SAs/DG was only

23 mv, which was lower than the other two catalysts (Figure 7i). Its tafel slope (25 mV·dec−1) was also slightly smaller than commercial Pt/C (30 mV·dec−1), indicating that the electron transfer in the HER process was faster in the presence of Pt SAs/DG (Figure 7j).

There is a strong Lewis acid-base interaction between the bulk metal and ammonia. With the assistance of ammonia gas, the metal is often easier to separate from the main body. It can be fixed on the substrate by solid diffusion method. However, this treatment method often requires a relatively cumbersome pickling procedure, and the requirements for corrosion resistance of the equipment will also increase. In addition, the addition of NH3 and the treatment of pickling wastewater will increase production costs and cause certain pollution to the environment. Taking into account the adverse effects of the above treatment methods, it is necessary to develop a simple and relatively environmentally friendly way to promote the preparation of SACS.

Excitingly, Qu et al. [90] used the dangling bonds of graphene oxide (GO) to directly capture Fe atoms in bulk Fe and isolate them on GO, thus simplifying the preparation process and reducing the generation of acid-base waste liquids. As shown in figure 7k, the GO slurry was uniformly injected into the Fe foam and dried at room temperature under flowing Ar for 12 h. Then, ethanol was added to the GO-Fe foam for ultrasonic treatment for 2 h. Due to the close contact between Fe foam and GO, the charge transfers between the two, forming a large amount of Feδ+.The coordination of Feδ+ with the dangling bonds on GO could produce Fe-O bonds, which pulled Fe atoms from the Fe lattice and anchor them on GO. Then, the ethanol was removed, washed by centrifugation and dried in vacuum, and finally Fe SAs/GO catalyst with 0~ +3 valence Fe atoms was prepared. The R-space of XANES showed that the high intensity peak of Fe SAS/GO at 1.5Å indicates that Fe SAS/GO had a strong Fe-O coordination (Figure 71). The fitting results of R-space spectrum for Fe SAs/GO showed that its local structure was a combination of one Fe atom and four O atoms (Figure 7m).Coincidentally, Ge et al. [94] also used bulk Fe as the precursor to prepare single atoms. The difference is that instead of dispersing Fe atoms on graphene oxide films (GOMs), they used GOMs as an atomic "filter" between the precursor and the carrier, allowing only single Fe atoms to penetrate. They heat-treated Fe foam to make Fe species diffused out in the form of particles. Fe single atoms passing through GOMs were anchored on the N-rich C carrier to form Fe SAs/N−C catalysts. In ORR, the half-wave potential of Fe SAs/N−C catalyst was larger (positive value) than commercial Pt/C, indicating that its catalytic activity was higher

Table 2 Summary of SACs by solid diffusion.

Synthesis methods

catalyst coordination models

Heating tempe-rature（℃）

Load (wt %)

Reaction type

Ref.

Nanoparticle

Pt-CHA-2 PtO3 650 0.21 wt Hydrogenation [79]

Pt/CeO2-rod --- 800 ~0.8 wt CO oxidation [80]

Pt/CeO2-polyhedra

--- 800 ~0.8 wt CO oxidation [80]

SE-Ni SAs@PNC.

Ni-3N and Ni-4N 900 --- CO2 electroreduction

[82]


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4 Liquid diffusion strategy Liquid diffusion includes electrochemical method and melten salt method. The electrochemical method is to electroplate a single metal ion on the electrode under a certain deposition potential, and then reduce it. The applied potential causes the metal ions to move directionally at a certain rate and provides electrons for the reduction of the metal ions. The concentration of precursor, scan rate and plating time can affect the loading of metal atoms on the carrier, and reaching a certain level can lead to the formation of nanoparticles [23]. The melting salt method refers to mixing the precursor, co-solvent and carrier and then heating it to a higher temperature to melt it, and then growing a single metal atom from the molten salt, and removing the template to obtain a single metal atom catalyst. During the synthesis process, the solid-solid reaction transforms into a solid-liquid reaction, which facilitates the movement of ions. Due to the addition of the co-solvent, the synthesis temperature can be reduced, and the co-solvent is also easy to remove and reuse.

4.1 Electrochemical method At present, the synthesis of single-atom catalysts is mainly achieved through template synthesis, thermal diffusion, and in-situ dispersion. These methods either have a cumbersome preparation process or require high temperature or high vacuum conditions, leading to higher production costs and a longer operation process. In contrast, electrochemical methods not only have lower production costs, but are also environmentally friendly. In the deposition process, the preparation of gradient deposits can be carried out by controlling the number of cycles and other parameters, so it is widely used in the synthesis of nanoparticles, clusters and monoatomic materials. In addition, this method can directly disperse metal atoms on the electrode substrate (For a two-electrode system, metal atoms can be deposited on the cathode or anode by applying different deposition potentials), and can be directly used in the corresponding electrocatalytic reaction without an adhesive. When depositing different metal atoms, the choice of deposition potential is often different. We have summarized this in Table 3.

s become single atoms

Pd-SAs Pd-N4 900 --- hydrogenation [84]

Bi SAs/NC Bi-N4 1000 0.2 wt CO2 electroreduction

[83]

CNT@NC@Ni SAs

Ni−N4 900 0.31 wt conversion of methane

[87]

Rh/CeO2 Rh−Ox-Cex 750 2 wt CO oxidation [95]

0.3Pt/Fe2O3-C800 Pt-Ox 800 1.8 wt methane oxidation [81]

PdPSA-CN Pd-Px 400 0.57 wt HER [96]

RuPSA-CN Ru-P4 400 0.78 wt HER [96]

RhPSA-CN Rh-P4 400 0.97 wt HER [96]

Solid migration

H-CPs Ni-Nx 1000 --- CO2RR [88]

Fe SAs/GO Fe-O4 500 6.7 wt ORR [90]

SAs/N-G Fe-O4 500 4.3 wt ORR [90]

Fe SAs/N−C Fe−N4 1000 0.73 wt ORR [94]

Pt SAs/DG Pt−C4 1100 2.1 wt HER [89]


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Fig 8 Pt SA-NT-NF synthesis schematic and performance characterization. a) A schematic diagram of the synthesis of Pt SA-NT-NF catalysts on working electrodes by electrochemical method. b) EDS map and HAADF image (insert) of NT, which show Pt exists in NT. c) AR-HAADF image of the NT. Highlight the Pd atoms with red circles. d) and f) mass activity (ƞ = 50 mV) and Stability test diagram (jHER = 100 mA cm-2) of Pt SA-NT-NF. e) The tafel slope of the NF, NT-NF, Pt SA-NT-NF, and Pt/C, where the Pt SA-NT-NF value is the smallest and the NF is the larges. (reproduced with permission from Ref. [17], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2019).

Zhang et al. [17] used electrochemical methods to prepare centimeter-level and stretchable Pt SA-NT-NF catalysts in neutral media. As shown in Figure 8a, a three-electrode system was adopted, with CoP-based nanotube arrays supported by a Ni foam (NT-NFs), Pt foil and SCE precursors as WE, CE and RE, respectively. At 25 °C, using N2-saturated neutral 1 M PBS (pH = 7.2) as electrolyte, and applying -1.5 V to -0.668 V (vs. SCE) deposition Potential at a sweep rate of 150 mv s-1. Pt atoms were deposited on NT-NFs in the form of +2 valence to form a Pt SA-NT-NF catalyst, which could be directly used as the cathode of HER without a binder. EDS showed the presence of Pt in NTs (Figure 8b), and the atomic resolution (AR) HAADF image showed that Pt atoms were atomically dispersed on NTs (Figure 8c), indicating that Pt atoms were isolated on NTs, not as Pt nanoparticles form existed. In the HER test, the mass activity of Pt SA-NT-NF was 70 Ag-1 (ƞ = 50 mV), which was about four times of commercial Pt/C (Figure 8d); The tafel slope was 30 mVdec-1, which was equivalent to the Pt/C catalyst (Figure 8e). In the HER stability test, it remained stable within 24 h, while the Pt/C showed a gradually decreased trend (Figure 8f). In addition to NT-NFs as the Substrate, Zhang et al. [23] used in-situ electrochemical methods and atomic capture strategies to successfully synthesize Mo2TiC2Tx-PtSA catalysts in a three-electrode system and acidic medium, using Pt foil as the precursor and MXene as the Substrate. During in-situ electrochemical deposition, Mo2TiC2Tx was

electrochemically peeled off into MXene nanosheets with Mo vacancies. At the same time, Pt atoms dissolved from the Pt foil into the acidic medium, moved to MXene under the action of the electric field, and then were captured by the Mo vacancies on it. MXene had a large specific surface area, rich surface chemistry and Mo vacancies formed by electrochemical exfoliation [97], which were conducive to fixing single Pt atom and improving the catalytic activity of MXene for hydrogen evolution reactions. In the HRE reaction, the mass activity of the Mo2TiC2Tx-PtSA catalyst was about 40 times that of the commercial Pt/C catalyst.

Ultra-micro electrodes have the advantages of high mass transfer rate and high current density, which provide a powerful means for the preparation and testing of electrocatalytic reaction materials. Zhou et al. [16] electroplated Pt atoms on the bismuth ultramicroelectrode (UME) according to multi-potential steps (Figure 9a). Using Bi UME with a radius of 253 nm as the substrate and 150 fM H2PtCl6 as the precursor, electroplating was turned on at -0.55 V, and the number of deposited Pt atoms was selected by applying different pulse times, and then electroplating was turned off at 0 V. A very low concentration precursor solution was used to control the mass transfer of Pt ions to the electrode, so that the Pt ions could reach the Bi UME substrate accurately at a certain diffusion rate. The synthesized Pt single-atom catalyst could be directly tested in the solution. Given the deposition time of 10 s (Figure 9c) and 20s (Figure 9b), the number of deposited Pt

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atoms follows Poisson Distributions. At a current density of 12.6 pA nm-2, the HER reaction was in a kinetically-controlled region (Figure 9d). After two years, Zhou et al. [24] once again used

electrochemical methods to successfully prepare gradient deposits of Pt (isolated single atoms, small clusters and nanoparticles) on Bi and Pb ultramicroelectrodes (UMEs).

Fig 9 Preparation of SACs by electrochemical method. a) Electrochemical synthesis and electrochemical measurements for a single platinum cluster on UMB carrier. b) Histogram of the number of Pt atoms under Poisson distribution and plating experiment. b) and c) the statistical chart of the number of Pt atoms drawn according to poisson distribution (bule) and plating experiments (orange) at plating times of 20 and 10 seconds, respectively. d) a Plots of Current density vs potential, where 1 to 9 represents the number of Pt atoms. (reproduced with permission from Ref. [16], © American Chemical Society 2017). e) Schematic diagram of Ir single atoms deposition at cathode (upper left) and anode (lower left) by electrochemical method, and Corresponding to a Plots of Ir mass loadings vs Ir concentration. f) Different kinds of single atomic catalyst were prepared by changing the carrier and metal precursor. (reproduced with permission from Ref. [15], © Springer Nature 2020).

Lately, Zhang’s group [15] also used electrochemical deposition

to deposit Ir single atom on the cathode and anode, respectively (figure 9e). In the acid electrolyte, a two-electrode system wa used for the electrochemical deposition process. Using Co0.8Fe0.2Se2@Ni foam as the cathode or anode and IrCl4 as the precursor, the single atom of Ir was deposited on the cathode or anode at a sweep rate of 5 mv s-1, and the deposition potential was −0.40 V to 0.1 V or 1.10 V to 1.80 V respectively. C-Ir1/Co0.8Fe0.2Se2 and A-Ir1/Co0.8Fe0.2Se2

single-atom catalysts with different electronic states were prepared. In order to prove the versatility of this method, they changed the metal M (M=Ru, Rh, Pd, Ag, Pt, Au, Fe, Co, Ni, Zn, V, Cr, Mn, and Cu) precursor and carrier (Co(OH)2, NC) were prepared according to the same steps, and more than 30 single-atom catalysts were successfully synthesized (as shown in Figure 9f, only some are listed).


ISSN 1998-0124 CN 11-5974/O4


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Fig 10 The synthesis, structure and performance characterization map of SACs prepared by specific electrochemical deposition method. a) Synthetic schematic of SACs. b) Schematic diagram of the self-limiting deposition process of metal atoms on TMD substrates. c) Model of single-atom metal anchored to TMD substrate. d) and e) DFT-optimization structure model and deposition location model of Pt atoms in the Pt-SAs/MoS2 catalyst, respectively. f) Charge density difference plots after anchoring Pt single atoms on MoS2 substrates. g) The Pt L3-edge XANES of Pt-SAs/MoS2 catalys. h) TOF curve of Pt-SAs/MoS2 and reference samples. the TOF values of Pt-SAs/MoS2 are higher than most reported HER catalysts in the test voltage range. i) Current density as a function of potential of Pt-SAs/MoS2 and reference samples. The inset shows that the mass activity of Pt-SAs/MoS2 is much higher than that of Pt/C.( reproduced with permission from Ref. [98], © Springer Nature 2020).

Different from the predecessors, Shi et al. [98] used the

underpotential deposition (CPU) process to introduce Cu atoms at specific positions on the substrate before depositing Pt single atoms, and then completely galvanically replaced Cu with Pt(II) at the deposition potential to produce atomically dispersed Pt-SAs/MoS2 (Figure 10a). After treatment like this, the surface restriction reaction automatically terminated the continuous growth of Pt atoms, allowing Pt atoms to be deposited individually at specific locations to prevent the formation of clusters and nanoparticles. They used different deposition potentials to deposit Cu and Pt atoms sequentially. In the three-electrode system, using 2 mM CuSO4 as the precursor, and applying a potential of 0.10 V in an acidic electrolyte, Cu-SAs/MoS2 was prepared first. Then it was transferred to the electrolyte containing 5 mM K2PtCl4 and stayed at the open circuit potential for more than 20 min, so that Pt(II) could completely replace Cu. This method was versatile and could change its support or precursor to prepare different types of single-atom catalysts (Figure 10c). Perform DFT calculation on Pt-SAs/MoS2, and the optimized structure model (Figure 10d) showed that Pt was deposited on top of Mo (Figure 10e). In the

UPD process, the metal-support interaction and activation energy (as shown in Figure 10b) provided favorable conditions for the self-terminating growth of atom-dispersed metals [99]. When single-atom Pt was immobilized on Pt-SAs/MoS2, the charge density difference figure (Figure 10f) incarnated the redistribution of the electronic structure, and XANES showed that Pt was partially oxidized ((Figure 10g). In HER, the TOF of Pt-SAs/MoS2 was 47.3 s−1 (100 mV), which was significantly higher than most reported single-atom catalysts (Figure 10h). Its overpotential was only ~59 mV (j10mA·cm

−2). After normalizing the Pt load, the mass activity of Pt-SAs/MoS2 was 17.14 A mg−1 (ƞ = 0.05 V), which was higher than commercial Pt /C 114 times (inset in the figure 10i).

As we all know, during the electrochemical deposition process, if the concentration of the precursor near the working electrode is high, as the deposition time increases, metal ions will continue to grow on the surface of the electrode, resulting in the formation of particles with larger diameters. When electroplating is carried out under under-potential, if the concentration of metal ions on the electrode surface reaches saturation, the growth of particles is restrained due to the self-limitation of the surface. Wang et al. [22]

Nano Res. 18

took a different approach, they placed the graphene oxide film (GOM) between the bulk Fe and the carrier as a "reducer" for Fe2+ ions, reducing the diffusion rate of Fe2+ ions and keeping the concentration of Fe2+ ions near the electrode at a low level, which prevent the Fe atoms on the carrier from continuing to nucleate under the deposition potential, and the Fe-SAs/NC catalyst was successfully prepared.

The electrochemical deposition method is simple and easy to promote, but the load is low. The method can accurately synthesize

single-atom catalysts by controlling the number of cycles, the deposition time, and the concentration of the precursor under the deposition potential. However, the nucleation and growth of metals in the electrochemical deposition process are not carried out in sequence. If the parameters are not properly controlled, it is easy to cause the formation of clusters or nanoparticles, and no single metal atom can be obtained. 4.2 Molten salt method

Fig 11 a) A synthesis scheme of Ni/TiO2 catalyst prepared by molten salt method. b) AC-STEM images of Ni/TiO2 catalyst. c) The high-resolution XPS spectra and The OV/OL ratio of TiO2 and Ni-a/TiO2 . d) Model of TiO2, TiO2-OV , Ni/TiO2 and Ni/TiO2-OV, where blue, red and white spheres represent Ti, O, Ni atoms, respectively. e) H2 yield at different reaction temperatures. f) The output of H2 under different Ni loads. (reproduced with permission from Ref. [100], © WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2020).

Unlike the catalyst, the promoter itself has no or little activity, but it can play an auxiliary role, thereby significantly improving the catalytic performance of the catalyst. Xiao and his colleagues [100] used the molten salt method (MSM) to load a small amount of Ni promoter on TiO2 nanoparticles to synthesize a single-atom promoter. The schematic diagram of the synthesis process was shown in Figure 11a. The solid salt (LiCl & KCl), TiO2 nanoparticles and Ni(NiCl2·6H2O) precursor were mixed and placed in a half-cap corundum crucible, heated to 773 K under N2 flow, and then maintained this temperature for 2 h. Subsequently, the salt began to melt into anions and cations, resulting in a strong polarizing force, resulting in the instability of the Ti-O bond, and causing oxygen ions on the surface of TiO2, which combined with the freely moving Ni2+ generated by the dissolution and dispersion of NiCl2 atoms to form Ni-O bonds, And then left oxygen vacancies on the adjacent surface of TiO2. The mixture was then cooled with N2, and finally washed thoroughly with water to remove the salt. AC-STEM proved that Ni or NiOx nanoparticles did not appear on Ni/TiO2 (Figure 11b). The XPS spectrum of Ni-a/TiO2 showed the formation of OV during the MSM process

(Figure 11c). DFT calculations found that the formation energy of OV on bare TiO2 was 5.46 eV, while the formation energy on Ni/TiO2 was only 4.0 eV (Figure 11d), which further supports the addition of Ni to promote the formation of Ov. In H2 evolution activity, the synthesis temperature and loading of Ni promoter affected the catalytic performance of the catalyst. When the heating temperature was 773 K, the obtained Ni-a/TiO2 had the largest H2 evolution rate (Figure 11e). When the loading of Ni was 0.46 wt% and 0.93 wt%, the H2 evolution rate was significantly higher than the other two groups. When the Ni loading was increased to 1.41 wt.%, the activity decreased obviously (Figure 11f). It showed that the existence of Ni single atom could improve the catalytic performance, but its loading was too high or too low will bring adverse effects on the catalysis [87]. The combination of single-atom promoter and TiO2 was closely related to the synthesis temperature. The low temperature may result in weaker binding capacity between the two. At high temperature, although it could promote the formation of the bond between the two and improve the stability of a single Ni atom, the single-atom co-catalyst tended to agglomerate due to its high dispersion and thermodynamic


Nano Res. 19

instability, resulting in a decrease in catalytic activity. Therefore, the temperature selection was the key to the preparation of sing-atomic promoters by the molten salt method.

It can be seen that the molten salt method can enhance the diffusion ability of the reactant in the molten salt medium and contribute to the high dispersion of the metal atoms [101]. Not only that, this method can also effectively control the morphology of the

product. Qiu et al. [102] added graphene oxide (GO) to molten salt, used KI as a shape inducer, and synthesized Pt Pd nanocubes with a core-shell structure on the GO surface. The above shows that the melten salt method has a simple synthesis process and a relatively mild reaction temperature, making it easier to realize industrialization.

Table 3 Summary of SACs by electrochemical method.

Synthesis methods

catalyst Loading (%) Deposition potential Reaction type Ref.

electrochemical method

C-Ir1/Co(OH)2 2.0 wt -0.4 ~ 0.1 V HER [15]

A-Ir1/Co(OH)2 1.2 wt 1.10 ~1.80 V OER [15]

Pt-SAs/MoS2 5.1 wt 0.1 V HER [98]

Bi UME --- -0.8 ~ -0.2 V HER [16]

PtSA-NT-NF 1.76 wt -1.5 ~ -0.668 V HER [17]

Pd0/GDY 0.20 wt --- HER [91]

Mo2TiC2Tx–PtSAc 1.2 wt -0.6 ~ 0 V HER [23]

Pt-2H-MoS2 1.1 at 1.9~2.3 V CO2RR [14]

400-SWNT/Pt 0.75 at 0~1.2 V HER [25]

Pt-SAs/C 1.26 wt --- HER [21]

Fe-SAs/N-C 0.52 wt 10 mA cm−2 CO2 reaction [22]

5 Conclusion In recent years, due to the continuous efforts of scientific researchers and the continuous development of characterization techniques, single atoms have gradually become "visualized", which has provided a huge boost to the development of single-atom catalyst synthesis methods. Different synthesis strategies capture and stabilize mononuclear metal species by changing the metal-support interaction. The strength of this interaction will affect the interface charge transfer, the electronic structure of the metal, etc., which in turn affects the stability and catalytic performance of SACs. If the force is too weak, a single metal atom is prone to diffusion and agglomeration, and then sintering occurs, resulting in a decrease in the number of catalytic active centers and a decrease in catalytic activity; If the force is too strong, a single metal atom will be loaded on the carrier in a higher oxidation state [103], and its stability will be reduced, resulting in a significant decrease in activity or even loss of activity. In this review, we discuss the synthesis methods of SACs by atomic diffusion strategy based on the diffusion form of the precursor, and explain how individual atoms are carried on the carrier.

At present, although a variety of synthetic strategies have been developed, some of which can manipulate and arrange atoms, but they have not been able to solve the problems of high stability and high load of SACs. Besides, the production cost required for the preparation of single-atom catalysts is relatively high, and it is currently difficult to popularize in industry. Advanced

characterization technology allows us to identify individual metal atoms, explore the synthesis and catalytic mechanisms of SACs, which greatly promotes the design of SACs and the development of SACs synthesis methods. Recently, we know from the reported literature[41, 81, 104-110] that reasonable selection of the support during the synthesis process and control of the reaction conditions are helpful to construct a special coordination structure, thereby enhancing the metal-support interaction, which can significantly increase the loading of metal single atoms . Although the load has increased, there is still a long way to go from basic research to industrial applications. In addition, during the synthesis process, as the load continues to increase, metal atoms are prone to migration and agglomeration, which leads to the formation of clusters or nanoparticles. This is one of the reasons why most synthesis methods enhance the stability of metal atoms at the expense of metal loading [15, 16, 20, 60, 111]. Therefore, it is still challenging to synthesize single-atom catalysts with high loading and high stability. How to synthesize efficient and inexpensive single-atom catalysts and how to maintain the stability of single atoms during the catalytic reaction still need us to explore.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21801015) and Beijing Institute of Technology Research Fund Program for Young Scholars (3090012221909).


Nano Res. 20

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