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The bottom‐up strategy, including coprecipitation, adsorption, and galvanic replacement methods, is the most common strategy to synthesize SACs. Firstly, mononuclear metal precursors are introduced onto the support surface. Then the product was dried and calcinated to remove organic ligands of the metal complexes. Finally, SACs are produced by reduction or activation [19].

Coprecipitation Method Coprecipitation seems to be the simplest method to prepare SACs. The precursors of the metal and the support should be soluble and could be coprecipitated by the precipitant at a certain pH value. The characteristics of the final catalysts, however, depend on many parameters including the order and the speed of the addition of the component solutions, the size of the droplets, efficient mixing, the temperature of the base solution, the pH value, and the aging time. Zhang and coworkers employed this method to fabricate single Pt atoms supported in iron oxide nanocrystallites (Pt₁/FeO_x), as demonstrated by aberration‐corrected scanning transmission electron microscopy (AC‐STEM) and extended X‐ray absorption fine structure (EXAFS) spectra (Figure 6.5) [16]. A series of SACs, M/TiO₂ (M = Pt, Pd, Rh, or Ru), were also synthesized by this method [24]. The SACs exhibit high photocatalytic hydrogen evolution performance compared with metal NPs. However, the main disadvantage of this approach is that some metal atoms can be buried at the interfacial regions of the support agglomerates and within the bulk of the support crystallites, thus compromising the effectiveness and efficiency of the SACs.

Figure 6.5 (a, b) HAADF‐STEM images of Pt₁/FeO_x. (c) The k³‐weighted Fourier transform spectra from EXAFS. (d) The normalized XANES spectra at the Pt L₃ edge of samples. Sample A refers to Pt₁/FeO_x with a Pt loading of 0.17%. Sample B refers to a similar catalyst with a Pt loading of 2.5 wt%.

Source: Qiao et al. 2011 [16]. Reproduced with permission of Springer Nature.

(See online version for color figure).

Adsorption Method The adsorption method, including impregnation, electrostatic adsorption, and adsorption assisted by other techniques, can also be used to synthesize SACs [50]. Generally, after the metal precursors are adsorbed on the support, the residual solution is removed, and the samples are then dried and calcined. The final metal loading and dispersion depend strongly on the nature of the anchoring sites on the support surfaces. This point should be considered for sample preparation.

Oxide supports are commonly used for preparing catalysts. For example, single Pt atoms were supported on θ‐Al₂O₃(010) surface by a wet impregnation method using alumina powder and chloroplatinic acid [38]. An impregnation–reduction method involves the impregnation of metal cations on oxides followed by on‐site reduction of metal cations, as exemplified by the preparation of singly dispersed Rh atoms supported on Co₃O₄ [51]. Gu et al. reported that single Pt₁ and Au₁ atoms can be stabilized by lattice oxygen on ZnO{1010} surface via an adsorption method [52]. High‐energy bottom‐up ball‐milling synthesis, a powerful method to break and reconstruct chemical bonds of materials with high efficiency, was used to synthesize a lattice‐confined single iron site catalyst embedded within a silica matrix [41, 53]. Liu et al. developed a room‐temperature photochemical strategy to fabricate stable atomically dispersed palladium–titanium oxide catalyst (Pd₁/TiO₂) with a Pd loading up to 1.5% [54]. Generally, the support, two‐atom‐thick TiO₂(B) nanosheets, was prepared by reacting TiCl₄ with ethylene glycolate. H₂PdCl₄ was added into a water dispersion of TiO₂(B) for the adsorption of Pd species. Then the mixture was irradiated by ultraviolet (UV) light to give a Pd₁/TiO₂ catalyst. However, metal oxide supports are not ideal for making supported catalysts used in electrocatalysis because metal oxides are generally insulators or semiconductors with low electron conductivities and they are often unstable under corrosive electrochemical conditions.

Zeolites appear to be promising supports for preparing SACs due to their well‐defined structures and high surface areas. Specifically, zeolites can provide effective voids to anchor individual metal atoms, thus maintaining the high dispersion of metals [42]. An atomically dispersed gold catalyst was prepared by the adsorption of an organogold precursor on zeolite NaY, and STEM images were used to determine the locations of isolated gold complexes in NaY [55]. However, this single‐atom Au(III)–O–NaY catalyst is unstable at 25 °C and 760 Torr in a flow reactor, losing ∼75% of their initial activity after 15 minutes in CO oxidation [55]. Flytzani‐Stephanopoulos and coworkers reported a successful approach to activate and stabilize atomic Au for the WGS reaction on zeolite and mesoporous silica ([Si]MCM‐41) materials with additional alkali ions [56].

Nitrides, carbons, and C₃N₄ have also been used to fabricate SACs. For example, single‐atom Pt deposited on TiN was prepared using an incipient wetness impregnation method, and this catalyst was used for electrochemical oxygen reduction, formic acid oxidation, and methanol oxidation [57]. Chen et al. prepared an Au SAC supported on polymeric mesoporous graphitic C₃N₄ by impregnation [58]. Wu and coworkers demonstrated an iced‐photochemical reduction strategy to construct atomically dispersed Pt species on mesoporous carbon [59]. Graphene can also be used as a support to obtain SACs. For instance, isolated Pt atoms were deposited on graphene nanosheets by atomic layer deposition (ALD), which involves graphite oxidation, thermal exfoliation, and chemical reduction [60]. As shown in Figure 6.6, firstly, the precursor MeCpPtMe₃ was injected to react with the adsorbed oxygen on the surface of graphene nanosheets, forming CO₂, H₂O, and hydrocarbon fragments. The limited supply of surface oxygen provides the self‐limiting growth necessary for ALD, creating a Pt‐containing monolayer. The subsequent oxygen exposure forms a new adsorbed oxygen layer on the Pt surface. The two processes form a complete ALD cycle. The Pt deposition can be precisely controlled by tuning the number of ALD cycles [60].

Figure 6.6 Schematic illustrations of Pt ALD mechanism on graphene nanosheets (GNSs).

Source: Sun et al. 2013 [60]. Reproduced with permission of Springer Nature.

(See online version for color figure).

The Galvanic Replacement Method Galvanic replacement is an electrochemical process that involves the oxidation of one metal, termed as a sacrificial template, by the ions of another metal with a higher reduction potential. When they contact each other at a liquid/solid interface, the sacrificial template will be oxidized and dissolved into the solution, while the ions of the second metal will be reduced, and the second metal will be doped onto the template surface [61]. Galvanic replacement is highly effective and versatile for the generation of a wide variety of metal nanostructures, due to its abilities to control the size and shape and to tune the composition, internal structure, and morphology of the resultant nanostructures [50].

This method has been applied to synthesize SACs in recent years. Sykes and coworkers showed that low‐concentration isolated Pt atoms on Cu(111) surface can be prepared by galvanic replacement on pre‐reduced Cu NPs (Figure 6.7a) [62]. They found that the isolated Pt atoms on Cu(111) surface can activate the dissociation and spillover of H to Cu. Sykes and coworkers also showed that such single‐atom Pt/Cu alloys can be used for C–H activation in methyl groups, methane, and butane [63]. An SAC consisting of isolated Rh atoms uniformly dispersed on the surface of VO₂ nanorods can also be synthesized by this method. Simply, Na₃RhCl₆ solution was injected into an aqueous solution containing VO₂ nanorods, and the mixture was stirred at 300 rpm at room temperature for one hour. In this process, Rh³⁺ ions were reduced to Rh⁺ by V⁴⁺ in the VO₂ nanorods. Isolated Rh atoms were then uniformly dispersed on the surface of the VO₂ nanorods, as seen by STEM (Figure 6.7b) [36].

Figure 6.7 (a) HAADF‐STEM and (b) magnified HAADF‐STEM images of Rh₁/VO₂.

Source: Wang et al. 2017 [36]. Reproduced with permission of John Wiley & Sons.