2 Atomically Precise Nanoclusters as Electrocatalysts
41
relationships between structure and properties [15, 16]. For example, Zhu et al. [17]
and Seoin et al. [18] reported the active site probing with Au catalysts of different
morphologies from the view of experimental and computational modeling, respectively. Despite the well-designed experiment, the non-atomically monodispersed
size of traditional nanomaterials significantly weakens the connection between the
structure and properties.
In the past decades, the synthesis strategy of atomically precise metal NCs has been
extensively investigated and a number of sizes of NCs between tens and hundreds
of atoms (equivalent diameters ranging from sub-nanometer to ~2.2 nm) have been
reported [1]. For applications as electrochemical catalytic materials, such NCs have
several distinctive features such as high surface area and unique surface structure
[19]. Besides, the atomic precision and crystal structure availability make metal NCs
a perfect system to bridge the structure and properties.
In this chapter, several works about metal NCs as electrochemical catalysts are
introduced with a focus on the atomic size effect, morphology effect, doping effect,
and charge effect. The computational techniques used in the catalytic mechanism
study are also summarized.
2.2 Synthesis and Structure Determination of Atomically
Precise Metal NCs
2.2.1 Synthesis of Metal NCs
Here, we illustrate the size-focusing synthesis and structure determination using
atomically precise Au 25 (SR) 18 NCs as an example. Larger NCs such as Au 133 (SR) 52
and Au 279 (SR) 84 can also be synthesized by the size-focusing method [20, 21]. In
the size-focusing method, a mixture of NCs with a controlled size distribution is first
prepared by carefully controlling the ratio of gold precursor and reduction agent as
well as other synthetic conditions. Then, the NCs mixture is subjected to size-focusing
under harsh conditions, under which the unstable NCs decompose or convert to
more stable ones. Eventually, only the most stable NCs can survive the size-focusing
process [22].
In the case of Au 25 [23, 24], the Au(III) salt is initially reduced to Au(I) by thiols
at 0 °C in the first step. The as-obtained Au(I)-SR complex is then reduced by adding
a NaBH 4 aqueous solution. Polydisperse NCs protected by thiolate are obtained
after the reduction process. During the following size-focusing process, it can be
observed from the evolution of the optical absorption spectra that the monodispersed
Au 25 NCs gradually become dominant, as shown in Fig. 2.1a. The mass spectrum
also illustrates the molecular purity of Au 25 (Fig. 2.1b).
41
relationships between structure and properties [15, 16]. For example, Zhu et al. [17]
and Seoin et al. [18] reported the active site probing with Au catalysts of different
morphologies from the view of experimental and computational modeling, respectively. Despite the well-designed experiment, the non-atomically monodispersed
size of traditional nanomaterials significantly weakens the connection between the
structure and properties.
In the past decades, the synthesis strategy of atomically precise metal NCs has been
extensively investigated and a number of sizes of NCs between tens and hundreds
of atoms (equivalent diameters ranging from sub-nanometer to ~2.2 nm) have been
reported [1]. For applications as electrochemical catalytic materials, such NCs have
several distinctive features such as high surface area and unique surface structure
[19]. Besides, the atomic precision and crystal structure availability make metal NCs
a perfect system to bridge the structure and properties.
In this chapter, several works about metal NCs as electrochemical catalysts are
introduced with a focus on the atomic size effect, morphology effect, doping effect,
and charge effect. The computational techniques used in the catalytic mechanism
study are also summarized.
2.2 Synthesis and Structure Determination of Atomically
Precise Metal NCs
2.2.1 Synthesis of Metal NCs
Here, we illustrate the size-focusing synthesis and structure determination using
atomically precise Au 25 (SR) 18 NCs as an example. Larger NCs such as Au 133 (SR) 52
and Au 279 (SR) 84 can also be synthesized by the size-focusing method [20, 21]. In
the size-focusing method, a mixture of NCs with a controlled size distribution is first
prepared by carefully controlling the ratio of gold precursor and reduction agent as
well as other synthetic conditions. Then, the NCs mixture is subjected to size-focusing
under harsh conditions, under which the unstable NCs decompose or convert to
more stable ones. Eventually, only the most stable NCs can survive the size-focusing
process [22].
In the case of Au 25 [23, 24], the Au(III) salt is initially reduced to Au(I) by thiols
at 0 °C in the first step. The as-obtained Au(I)-SR complex is then reduced by adding
a NaBH 4 aqueous solution. Polydisperse NCs protected by thiolate are obtained
after the reduction process. During the following size-focusing process, it can be
observed from the evolution of the optical absorption spectra that the monodispersed
Au 25 NCs gradually become dominant, as shown in Fig. 2.1a. The mass spectrum
also illustrates the molecular purity of Au 25 (Fig. 2.1b).