Atomic-Level Doping of Metal Clusters - Accounts of Chemical

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Atomic-Level Doping of Metal Clusters Published as part of the Accounts of Chemical Research special issue “Toward Atomic Precision in Nanoscience”. Atanu Ghosh,† Omar F. Mohammed,‡ and Osman M. Bakr*,† KAUST Catalysis Center (KCC) and ‡Division of Physical Science and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia

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CONSPECTUS: Atomically precise noble metal (mainly silver and gold) nanoclusters are an emerging category of promising functional materials for future applications in energy, sensing, catalysis, and nanoelectronics. These nanoclusters are protected by ligands such as thiols, phosphines, and hydride and have sizes between those of atoms and plasmonic nanoparticles. In metallurgy, the properties of a pure metal are modified by the addition of other metals, which often offers augmented characteristics, making them more utilizable for real-life applications. In this Account, we discuss how the incorporation of various metal atoms into existing protected nanoclusters tunes their structure and properties. The process of incorporating metals into an existing cluster is known as doping; the product is known as a doped cluster, and the incorporated metal atom is called a dopant/foreign atom. We first present a brief historical overview of protected clusters and the need for doping and explain (with examples) the difference between an “alloy” and a “doped” cluster, which are two frequently confused terms. We then discuss several commonly observed challenges in the synthesis of doped clusters: (i) doping produces a mixture of compositions that prevents the growth of single crystals; (ii) doping with foreign atoms sometimes changes the overall composition and structure of the parent cluster; and (iii) doping beyond a certain number of foreign atoms decomposes the doped cluster. After delineating the challenges, we review a few potential synthetic methods for doped clusters: (i) the co-reduction method, (ii) the galvanic exchange method, (iii) ligand-induced conversion of bimetallic clusters to doped clusters, and (iv) intercluster reactions. As a foreign atom is able to occupy different positions within the structure of the parent cluster, we examine the structural relationship between the parent clusters and their different foreign-atom-doped clusters. We then show how doping enhances the stability, luminescence, and catalytic properties of clusters. The enhancement factor highly depends on the number and nature of the foreign atoms, which can also alter the charge state of the parent cluster. Atomic-level doping of foreign atoms in the parent cluster is confirmed by high-resolution electrospray ionization and matrixassisted laser desorption ionization mass spectrometry techniques and single-crystal X-ray diffraction methods. The photophysical properties of the doped clusters are investigated using both time-dependent and steady-state luminescence and optical absorption spectroscopies. After presenting an overview of atomic-level doping in metal clusters and demonstrating its importance for enriching the chemistry and photophysics of clusters and extending their applications, we conclude this Account with a brief perspective on the field’s future.

1. INTRODUCTION

These nanoclusters are designated by a molecular formula, such as [Mx(SR)y]q (where x, y, and q correspond to the total number of metal atoms and ligands and the charge state, respectively),6 and span the size scale between atomic and plasmonic nanoparticles. However, despite their structural appeal, the generally poor stability and catalytic properties of nanoclusters put them out of reach of many practical applications. Improving their stability and diverse properties is now the major priority for the field of cluster chemistry.

Metal nanoparticles (NPs) stabilized by organic ligands are generally designated by their sizes (typically in nanometers).1 Precise assignment of the number of metal atoms and ligands is not possible for such systems, and the nature of the bonding between the metal atoms and ligands is not well understood because of the unavailability of crystal structures. Consequently, surface modification of these nanoparticles to realize various applications has appeared challenging. An exception to this rule are noble metal nanoclusters protected by ligands such as thiols, phosphines, and hydride, which are an emerging class of materials with atomically precise numbers of ligands and metal atoms and well-defined, accessible structures.1−5 © XXXX American Chemical Society

Received: August 14, 2018

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DOI: 10.1021/acs.accounts.8b00412 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research Scheme 1. Schematic Illustration of Metal Doping and Its Effects on Ag25(SR)18

cluster properties (see, e.g., Scheme 1). Specifically, doping can induce charge stripping and structural changes as well as modulate the optical and electronic states, reactivity, and catalytic activities of the parent clusters. Those effects are sensitive to the number and nature of the dopant metal atoms. For example, the photoluminescence intensity of the Ag25(SR)18 cluster increases ∼25-fold when doped with Au atoms, whereas it remains almost the same when doped with Pd (Scheme 1).18 Finally, we conclude with a brief discussion of the future prospects of the field.

In metallurgy, judiciously designed alloys comprising a mixture of different metals usually outperform pure metals. For instance, the hardness and corrosion resistance of copper can be immensely enhanced by mixing it with zinc and other metals, leading to the well-known alloy brass. Our daily lives and the technological development of society depend on the applications of such alloy materials. Perhaps taking its inspiration from traditional metallurgy, the cluster community introduced metals into existing nanoclusters in order to enhance their stability and modulate their diverse properties.7−27 These materials became known as “doped” clusters. While the monolayer-protected metal NP field was ignited by the work of Brust and Schiffrin,28 the subsequent detection of atomically defined NP species by Whetten and co-workers29 could be considered the major starting point for the field of atomically precise metal clusters. Later, a large number of welldefined monometallic clusters and their foreign-atom-doped bimetallic cluster counterparts were successfully synthesized.1−4 Initially, the formation of doped clusters was confirmed by matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) mass spectrometry (MS) measurements.1,2,6,29 Foreign-atom-doped trimetallic30 and tetrametallic31 clusters were also observed during the MS measurements. Single-crystal X-ray diffraction (SC-XRD) measurements played a significant role in determining the exact positions of the foreign atoms in the parent cluster.16−22,32−35 To date, only a handful of doped clusters have had their structures fully elucidated, while the large majority remain unexplored because of the absence of SC-XRD data. Thus, the synthesis and crystallization of these doped clusters remain an essential challenge to unlocking their properties and potential applications. In this Account, we mainly discuss the chemistry of doped clusters. We evaluate the challenges in the synthesis of doped clusters and highlight the key role of controlling the doping process with foreign atoms in the successful production of doped clusters. We overview the four potential synthetic methods for doped clusters: (i) the co-reduction method, (ii) the galvanic exchange method, (iii) ligand-induced conversion of bimetallic clusters to doped clusters, and (iv) intercluster reactions. Subsequently, we show how doping affects various

2. DOPED VERSUS ALLOY CLUSTERS The existing multimetallic clusters can be classified into two categories. When one atom or multiple atoms of an existing cluster are replaced by atom(s) of another metal, the resulting product is called a “doped” cluster. For example, a Pt atom replaces one of the silver atoms in the well-known cluster [Ag29BDT12(PPh3)4]3− (BDT = 1,3-benzenedithiol) and produces [Ag28Pt1BDT12(PPh3)4]4−, which is known as the mono-Pt-doped Ag29 cluster.20 Doping with multiple foreign atoms is also well-known in cluster science. In this case, the doped cluster can be bimetallic, trimetallic, or tetrametallic depending upon the nature of the foreign atoms. The cluster may be called an “alloy” when it contains two or more different metal atoms, and unlike a “doped” cluster, the existence of a parent cluster is not mandatory. For instance, [Ag28Cu12(2,4DCBT)24]4− (2,4-DCBT = 2,4-dichlorobenzenethiol) is an alloy cluster, while the parent [Ag40(2,4-DCBT)24] cluster is unknown.36 Therefore, it can be generalized that all doped clusters are alloy clusters, but not all alloy clusters are doped clusters. The synthesis of doped clusters is more stringent than that of alloy clusters. For the synthesis of alloy clusters, there is no restriction on the number of metal atoms in the cluster composition: the only requirement is the presence of atoms of two or more different metals in the cluster composition. However, the synthesis of doped clusters is quite challenging, as the total number of metal atoms and ligands in the parent cluster should be preserved after doping. In the next section, we highlight methods for the successful creation of doped clusters. B


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Accounts of Chemical Research

Figure 1. Doping-induced charge stripping of the (A) Au38(SC6H13)24, (B) Au25(SC2H4Ph)18, (C) Ag25(2,4-DMBT)18, and (D) Ag29(BDT)12(PPh3)4 clusters.

3. SYNTHESIS OF DOPED CLUSTERS The development of nanocluster materials depends on the availability of nanoclusters with diverse structures and properties. Therefore, the synthesis of doped clusters plays a crucial role in the development of this area. However, doping poses a few challenges during synthesis, often resulting in a mixture of compositions that prevent the growth of single crystals24,37 or changing the overall composition and structure of the parent cluster,38 and in some cases the doped cluster decomposes.14 Described below are some of the most promising methods for synthesizing doped clusters.

result of the difference in electrochemical potential, which helps in the formation of the Au-doped Ag25 cluster, Ag24Au. Notably, when we tried to synthesize Ag24Au by the coreduction method, the reaction produced a mixture of compositions, [Ag25−xAux(2,4-DMBT)18]− (x = 1−8).18 The choice of metal precursors is very crucial for such a process. Mono-Cd- and -Hg-doped Au25 clusters, Au24Cd39 and Au24Hg,32 were also synthesized by this method. 3.3. Ligand-Induced Conversion of Bimetallic Clusters to Doped Clusters

In this procedure, one cluster spontaneously converts to another cluster in the presence of a second ligand. The final product is exclusively protected with the second ligand. The ligand-induced conversion of one monometallic cluster to another monometallic cluster is well-known.1 Recently, however, the ligand-induced conversion of one bimetallic cluster to a doped cluster was reported for silver clusters. Mono-Pt-doped Ag29 clusters protected with different ligands, namely, [Pt 1 Ag 28 (BDT) 12 (PPh 3 ) 4 ] 4− and [Pt 1 Ag 28 (SAdm)18(PPh3)4]2+ (S-Adm = adamantanethiol), were synthesized by this procedure.20,21,23 The bimetallic cluster [Pt1Ag24(2,4-DMBT)18] was converted to the doped cluster [Pt1Ag28(S-Adm)18(PPh3)4]2+ in the presence of a second thiol, S-Adm.21 Similarly, in the presence of the BDT ligand, the bimetallic cluster [Pt2Ag23Cl7(PPh3)10] was converted to the doped cluster [Pt1Ag28(BDT)12(PPh3)4]4−.23 Conversion of [Pt1Ag28(S-Adm)18(PPh3)4]2+ to [Pt1Ag28(BDT)12(PPh3)4]4− was also observed when the BDT ligand was added to a solution of the former cluster.20

3.1. Co-reduction Method

In this one-pot synthetic method, metal precursors of the parent material and the foreign metal are reduced simultaneously using a reducing agent in the presence of the protecting ligands. Negishi et al.8 synthesized a Pd-doped Au38 cluster, Pd2Au36(PET)24 (PET = phenylethanethiol), by the co-reduction of a gold(III) chloride (HAuCl4) and sodium tetrachloropalladate (Na2PdCl4) in the presence of the PET ligand using the reducing agent NaBH4. A large number of doped clusters, including Ag32Au12,16 Pt1Au24(PET)18,15 Pd1Au24(SC12H25)18,7 and [M1Ag24(2,4-DCBT)18]2− (M = Pd, Pt),17 have been synthesized via this one-pot synthetic method. 3.2. Galvanic Exchange Method

Here the metal precursor of the foreign material is added to the solution of the parent cluster in a controlled manner. A monogold-doped Ag25 cluster, Ag24Au1(2,4-DMBT)18 (DMBT = 2,4-dimethylbenzenethiol), was synthesized by this method, where a gold precursor salt, AuPPh3Cl, was added to a solution containing Ag25(2,4-DMBT)18 clusters.18 In this process, the Ag atoms of the Ag25 cluster reduce the Au+ ions to Au0 as a

3.4. Intercluster Reactions

The reaction between two different types of metal clusters in solution produces doped clusters through a metal-exchange C


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Figure 2. Construction of the [Ag24Au1(2,4-DMBT)18]− cluster starting with a gold atom. (A−C) Formation of the centered icosahedral core (Ag12Au1). (D) Structure of the six V-shaped staple motifs (Ag2S3). (E) Centered icosahedral core covered with six Ag2S3 staple motifs. (F) Total structure of [Ag24Au1(2,4-DMBT)18]− with the counterion. Legend: silver-white, Ag; red, Au; purple, P; yellow, S; gray, C. Reproduced with permission from ref 18. Copyright 2016 Wiley-VCH.

Doping-induced charge stripping of Au38, Au25, Ag25, and Ag29 clusters is illustrated in Figure 1.

pathway. The iridium-doped Au25 cluster Au22Ir3(SR)18 was produced by the intercluster reaction between Ir9(SR)6 and Au25(SR)18 clusters.40 Doped clusters of Ag25, Ag29, Ag44, and Ag51 were also synthesized according to this method.41 Here controlled doping is possible by maintaining the concentrations of the clusters during the reaction.

4.2. Crystal Structure

Mass spectrometry generally confirms the formation of doped clusters. However, it does not provide information about the position of the foreign metal atom in the structure of the parent cluster. SC-XRD plays an important role in understanding the structural relation between the doped cluster and the parent cluster. In this respect, Au25(SR)18, which holds the record for the largest number of structurally elucidated doped analogues, offers the most abundant insights in the structure− property relationship present in gold clusters.32,33,37,39,43 Among the reported silver clusters, Ag25(SR)18 is the only cluster that has an isoelectronic and isostructural analogue in gold, i.e., Au25(SR)18.18,44 Similar to the Au25(SR)18 cluster,45 Ag25(SR)1844 contains a centered icosahedral Ag13 core. This centered icosahedral core has 20 triangular faces. Of the remaining 12 non-icosahedral silver atoms, nine occupy triangular face centers of the icosahedron. Surprisingly, the remaining three non-icosahedral silver atoms face away from the triangular face centers. These arrangements differentiate the structure of Ag25 from that of the Au25 cluster, where all 12 non-icosahedral silver atoms occupy triangular face centers of the icosahedron. Alternatively, the structure can be visualized such that the centered icosahedral core, Ag13, is covered with six Ag2S3 staple motifs (identical to Au25). This structural likeness is responsible for some of the similarities between the optical properties of Ag25 and Au25 clusters.18,44,45 The surface of the icosahedral core in the monocadmiumdoped Au24Cd1(SR)18 cluster39 is occupied by the cadmium atom, whereas one of the Au atoms of the ligand motif (Au2S3)

4. EFFECTS OF DOPING 4.1. Charge State

Doping-induced charge stripping is an interesting phenomenon in doped cluster chemistry. The charge stripping phenomenon depends on the parent cluster and the type of dopant metal atoms. In the case of the Au38(PET)24 cluster, the charge state changes from 0 to 2− when the cluster is doped with two Pt atoms, whereas the charge state remains unchanged following Pd doping.9,42 For the Au25(PET)18 cluster, the charge state is not altered upon Ag doping but changes from 1− to 0 upon doping with platinum and mercury atoms.15 The charge state of the Au18(SC6H11)14 cluster also remains identical when silver atoms replace three of the gold atoms.22 Similar to the Au25(PET)18 and Au18(SC6H11)14 clusters, the charge states of the Ag25,18 Ag2919 , and Ag50(Dppm)6(TBBM)30 (Dppm = bis(diphenylphosphino)methane; TBBM = 4-tert-butylbenzyl mercaptan)24 clusters do not change upon doping with gold atoms. However, the charge states of Ag25 and Ag29 clusters change upon Pt atom doping. The charge state of Ag25 is converted from 1− to 2− upon doping with Pt atoms,17 whereas the charge state of Ag29 clusters changes from 3− to 4− (i.e., [Pt 1 Ag 28 (BDT) 12 (PPh 3 ) 4 ] 4− ) 20 and 2+ (i.e.[Pt 1 Ag 28 (SAdm)18(PPh3)4]2+)21 upon doping with a single Pt atom. D


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Figure 3. (A1−A3) Construction of Ag29S24P4 in the [Ag29(BDT)12(PPh3)4]3− cluster. (B1−B3) Construction of Pt1Ag28S18P4 in the [Pt1Ag28(SAdm)18(PPh3)4]2+ cluster. The hydrogen and carbon atoms have been omitted for clarity. Legend: blue, Ag; pink, Pt; red, S; green, P. (A1−A3) Adapted from ref 46. Copyright 2015 American Chemical Society. (B1−B3) Adapted with permission from ref 21. Copyright 2017 Royal Society of Chemistry.

is substituted by a Hg atom in Au24Hg1(SR)18.32 The center of the icosahedron is occupied by a Pd atom in Au24Pd1(SR)18.33 Thus, we expect that in Ag25(SR)18, which is isostructural to Au25(SR)18, the foreign atom should be able to occupy three different locations: the center and the surface of the icosahedron and the ligand motif (Ag2S3). The gold atom in the Ag24Au1(SR)18 cluster occupies the center of the icosahedron. 18 Unlike the Ag 25 (SR) 18 cluster, the Ag24Au1(SR)18 cluster features 12 non-icosahedral silver atoms arranged at triangular face centers of the Ag12Au1 icosahedron (identical to the Au25 cluster). Similar to Ag25(SR)18 and Au25(SR)18, the icosahedral Ag12Au1 core in this structure is also protected by six Ag2S3 staple motifs. The construction of the Ag24Au1(SR)18 cluster starting with a gold atom is shown in Figure 2. The Agicosahedron−S and Agstaple motif −S bond lengths are shorter in Ag24Au1(SR)18 than in Ag25(SR)18, which implies that the strength of the Ag−S bond increases as a result of gold doping. These structural relations can be correlated with their stabilities. The golddoped cluster is more stable than the parent Ag25(SR)18 cluster. Such enhanced stability due to doping is also known for other doped clusters. The doped cluster [Ag24Pd1(SR)18]2− has an icosahedral Pd1Au12 core. The center of the icosahedron is occupied by the Pd atom, and the core is covered by six staple motifs (−S−Ag−S−Ag−S−).17 Although the Pd1Ag12 core structure is almost the same as those of the previously discussed clusters, the surface structure (the bond angle in the six Ag2S3 staple motifs) is quite different from that of the Au25(SR)18 cluster, the implications of which are discussed elsewhere in detail.17 Similar to the Ag25 and Au25 clusters, the Ag29 cluster also contains a centered icosahedral core.44−46 In the case of the Ag29 cluster, the arrangements of the non-icosahedral silver atoms are entirely different from those in the Ag25 and Au25 clusters. The centered icosahedral core (Ag13) in Ag29 is covered with a Ag16S24P4 shell that consists of two different

types of motifs: (i) four Ag3S6 crown motifs and (ii) four Ag1S3P1 motifs (Figure 3A1−A3). Details of the structures are discussed elsewhere.46 The structure of the [Pt1Ag28(BDT)12(PPh3)4] cluster is almost identical to that of the Ag29 cluster.20 Here the Pt atom occupies the center of the icosahedron. In the case of [Ag29−xAux(BDT)12]3− (x = 1− 5), we successfully crystallized only one composition, [Ag28Au1(BDT)12]3−, which has a structure identical to that of the [Pt1Ag28(BDT)12(PPh3)4]4− cluster.19 The Au atom occupies the center of the icosahedron. Other compositions, namely, [Ag29−xAux(BDT)12]3− (x = 2−5), were noncrystallizable, as they contained a mixture of alloy clusters with possible isomers that retarded single-crystal formation. The positions of the next four Au atoms in the structure of the Ag29 cluster were predicted by 31P NMR spectroscopy, which indicated that the Au atom most likely occupies the four phosphine bridging sites.19 Both the core and shell structures of the monothiolprotected Ag28Pt1 cluster, [Pt1Ag28(S-Adm)18(PPh3)4]2+,21 are completely different from those of the previously discussed BDT-protected Ag29 and Ag28Pt1 clusters. Unlike the centered icosahedral Pt1Ag12 core in [Pt1Ag28(BDT)12(PPh3)4], Pt1Ag12 features a face-centered cubic (FCC) shape (Figure 3 A1,B1). The FCC core is stabilized by the Ag16S18P4 shell, which consists of four identical Ag4S6P1 motifs sharing six silver atoms (Figure 3B1−B3). A comparison of the structures of [Ag29(BDT)12(PPh3)4]3− and [Pt1Ag28(S-Adm)18(PPh3)4]2+ clusters is shown in Figure 3. It is worth noting that the structure of the parent cluster dictates to a large extent its possible doping states. For example, unlike the previously discussed clusters, Ag44(SR)30 contains a hollow Ag12 icosahedral core,16 which may explain its main reported crystal structure of the doped derivative, the 12-atom-doped Ag32Au12(SR)30 cluster16,47 (in contrast to the well-known structures of the monodoped clusters of Ag25, Au25, and Ag29). In the same vein, the three-silver-atom-doped E


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Figure 4. Comparison of the photophysical properties of the Ag25, Ag24Au1, and Ag24Pd1 clusters: (A) UV−vis, (B) PL, and (C) TA spectra. Insets: (A) visible-light photographs of the solutions; (B) normalized photoluminescence spectra. Reproduced with permission from ref 18. Copyright 2016 Wiley-VCH.

Figure 5. (A) Photoluminescence enhancement of the [Ag29−xAux(BDT)12(PPh3)4]3− cluster due to Au (mmol %) doping: (i) UV light photograph of the doped cluster; (ii) corresponding luminescence spectra. The luminescence intensity highly depends on the amount of gold doping. (B) Comparison of the TA spectra of pure and Au-doped (40 mol %) Ag29 clusters. (C) GSB recovery kinetics of pristine Ag29 and 20% and 40% Au-doped clusters on the millisecond and (inset) nanosecond time scales. Solid lines represent exponential fits. Reproduced with permission from ref 19. Copyright 2016 Wiley-VCH.

modification. Controlled doping is considered the most promising method, as it maintains the overall composition of the cluster. The PLQY of the rod-shaped Au25 cluster is only 0.1% but increases to ∼40% upon doping with silver atoms, making the cluster a good candidate for cell imaging.14 The PLQY of the doped Au25−xAgx cluster strongly depends on the number of Ag atoms. For x = 1−12, the PLQY increased to merely

Au15Ag3(SC6H11)14 cluster is the only known doped derivative of Au18(SC6H11)14.22 4.3. Photophysical Properties

The photoluminescence quantum yields (PLQYs) of most ligand-protected clusters are very low, preventing their use in luminescence-based applications such as cell imaging. Two strategies have been adopted for enhancing the PLQY: (i) doping with foreign metal atoms and (ii) ligand shell F


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Accounts of Chemical Research Scheme 2. Reactions of (i) the [PdAg24(SR)18]2− Cluster and (ii) the [PtAg24(SR)18]2− Cluster with AuPPh3Cl

∼0.21%, whereas for x = 13 the PLQY reached ∼40%. This type of luminescence enhancement was also observed for its isoelectronic and isostructural silver cluster, Ag25. In the case of the Ag25 cluster, the nature of the foreign metal plays a crucial role in tuning the optical properties. The color of the parent solution changed from yellow to yellowish-orange when the Ag25 cluster was doped with Pd. However, the color of the parent solution completely changed to green upon Au doping (Figure 4A).18 Unlike Pd doping, Au doping caused considerable blue shifts in both the absorption (major peak) and photoluminescence spectra (Figure 4A,B).18 The luminescence of the Ag25 cluster is enhanced by a factor of ∼25 upon doping with a gold atom.18 The stabilization of charge states in the lowest unoccupied molecular orbital (LUMO) of the Au-doped alloy cluster is responsible for this enhancement. In other words, foreign-atom doping can modulate the electronic structure of the Ag25 cluster, as reflected by the excited-state behavior. Comparison of the transient absorption (TA) spectra of the parent and doped clusters shows that the ground-state bleaching (GSB) signal marginally shifted for Ag24Pd, whereas a large shift was observed for Ag24Au (Figure 4C).18 This observation is consistent with their steady-state absorption behavior. The relaxation dynamics shows that the time constant for the excited-state decay of Ag25 is 1.1 μs, which is due to the ligandto-metal charge transfer (LMCT) state. The values increased to 1.3 and 1.8 μs upon doping with Pd and Au, respectively. The modified electronic structure arising from foreign-atom doping is responsible for this observation. Similarly, the relaxation dynamics of doped Au25 clusters, M1Au24 (M = Cd, Hg, Pd, Pt), also depends on the nature of the foreign atoms, as discussed in detail elsewhere.48 The above optical effects cannot be generalized. For instance, while in the case of another silver cluster, Ag29, we also observed a doping-induced alteration of its optical properties, doping yielded somewhat different effects from those observed in the previously discussed cluster. For example, the luminescence maximum is red-shifted for the

Au-doped Ag29 cluster (Figure 5A), whereas a blue shift was observed for the Au-doped Ag25 cluster (Figure 4B).18,19 In both cases, the major absorption peak is blue-shifted. We found that the luminescence intensity also depended on the number of doped gold atoms. The intensity increased as the percentage of Au precursor in the sample increased (Figure 5A). A PLQY enhancement from 0.9 to 30% was achieved during this doping process.19 The origin of the PLQY enhancement of the Au-doped Ag29 cluster was investigated by TA spectroscopy.19 The GSB signal (445 nm) in the TA spectra is blue-shifted for the Au-doped Ag29 cluster, which is consistent with their ground-state absorption profiles (Figure 5B). The shape of the spectra in the range of 485−800 nm also changed during Au doping (Figure 5B). Unlike the undoped Ag29 nanocluster, which showed a single long-lived LMCT time constant (300 ns) upon fitting of its GSB traces (445 nm), the Au-doped clusters showed two time constants, with the longer time constant substantially increasing from 612 to 890 ns in 20 and 40% Au-doped clusters, respectively (Figure 5C). This result corroborated the hypothesis that the Au dopant atoms perturb the electronic relaxation and dynamics of the nanocluster. Pyo et al.49 showed that rigidification of the surface ligands also plays an important role in enhancing the luminescence of gold clusters. We have observed that rigidification also improved the PLQY of silver clusters, such as Ag29.20 Here the tetraoctylammonium (TOA) cation was used to achieve rigidification. The PLQY enhancement factor is 5 times greater for the Ag28Pt1 (dithiol protected) cluster than for the parent Ag29 cluster. ESI-MS showed that the 4− charge state of Ag28Pt1 allowed it to form an adduct with two TOA cations, whereas Ag29 formed an adduct with only one TOA cation. The presence of a greater number of TOA cations is responsible for the higher QY enhancement factor of Ag28Pt. Surprisingly, the PLQY remained unchanged for the monothiol-protected Ag28Pt1 cluster under identical conditions. This observation can be explained by comparing their crystal structures, as discussed elsewhere.20 Finally, it has been G


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Figure 6. (A) Effect of Pd atom doping on the catalytic oxidation of benzyl alcohol. Reproduced from ref 13. Copyright 2012 American Chemical Society. (B) Doping-induced enhancement of the catalytic performance of the Ag25 cluster. Reproduced with permission from ref 53. Copyright 2018 Wiley-VCH.

The structural chemistry of tri- and tetrametallic doped clusters has not reported to date; such clusters have only been observed by mass spectrometry (usually containing a mixture of compositions).30,31 The lack of atomic-level purity is responsible for the retarded progress of this area of the field, as it practically rules out the growth of single crystals. While the community has recognized the potential role of noble metal clusters as fluorescence probes and catalysts and hence has made tremendous progress in enhancing the luminescence and catalytic activity of clusters through doping, not much effort has been dedicated to enhancing their magnetic properties. Without doping, the magnetic response of noble metal clusters is generally weak. Doping with ferromagnetic metals such as Fe, Co, and Ni is a promising pathway to introduce magnetic properties into such clusters. It is anticipated that doping clusters with such metals will require the development of new synthetic routes other than those currently known for doping. During doping, different foreign metal atoms prefer to occupy different positions in the parent cluster. The reason behind this preferential occupancy is not yet understood. However, if uncovered, it will have major applications in tuning the catalytic and surface reactivity of clusters. Indeed, there are still many avenues that remain to be explored before the community achieves a firm grasp of atomically precise doping of clusters with control over the nature and number of the foreign metal atoms.

reported that metal doping can affect the charge transfer dynamics at the interface of silver nanoclusters. For instance, femtosecond TA experiments have demonstrated that the charge transfer at the interface between the Ag29 cluster and a porphyrin molecular acceptor can be tuned by doping Ag29 with Au.50 4.4. Reactivity

Negishi and co-workers showed that the ligand exchange reactivity of the [Au25(SC12H25)18]− cluster increases when the cluster is doped with a single palladium atom.25 Bürgi and coworkers reported that the surface flexibility of the Au38(SR)24 cluster increased upon doping with multiple silver or palladium atoms.51 We observed that the reactivity of doped clusters with metal salts depended on the nature of the foreign metal.52 When the [Pd1Ag24(SR)18]2− cluster is reacted with the AuPPh3Cl salt, the central Pd atom of the cluster is replaced by the gold atom, and the bimetallic doped cluster [Au1Ag24(SR)18]− is produced (Scheme 2i). In contrast, under identical experimental conditions, the [Pt1Ag24(SR)18]2− cluster produces the trimetallic doped clusters [Ag24−xAuxPt (SR)18]2− (x = 1−2), where the central Pt atom remains unexchanged (Scheme 2ii). The reason for these observations is discussed elsewhere.52 4.5. Catalytic Activity

In the previous section, we discussed how doping modulates the electronic structure of the parent cluster, which affects the catalytic properties of such materials. Tsukuda and co-workers showed that the percent conversion for benzyl alcohol oxidation increased from 22 to 74% when the parent Au25 cluster was doped with a single Pd atom (i.e., Au24Pd1) (Figure 6A).13 Most recently, Zhu and co-workers showed that foreignatom doping enhances the catalytic performance of the Ag25 cluster and that the enhancement factor depends on the nature of the foreign metal atom (Figure 6B).53

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Omar F. Mohammed: 0000-0001-8500-1130 Osman M. Bakr: 0000-0002-3428-1002 Notes

5. CONCLUDING REMARKS AND FUTURE PROSPECTS Atomically pure doped clusters have been synthesized for only a few classes of clusters, whereas for most clusters atomic-level purity is not yet achieved during doping. Such purity is very crucial for producing single crystals of doped materials. However, despite this limitation, the current small set of doped clusters have brought substantial new insights in the understanding of cluster chemistry and led to a leap in property diversification of clusters.

The authors declare no competing financial interest. Biographies Atanu Ghosh received his Ph.D. from IIT Madras under the supervision of Prof. T. Pradeep. He is currently a postdoctoral fellow at KAUST. His research is focused on cluster-based catalysis. Omar F. Mohammed is an assistant professor in the Division of Physical Sciences and Engineering at KAUST. He built and developed the first and second generations of scanning ultrafast electron H


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microscopes at Caltech and KAUST, respectively. His research activities are focused on the development of highly efficient solar cells with the aid of cutting-edge laser spectroscopy and electron microscopy. Osman M. Bakr holds a B.Sc. in Materials Science and Engineering from MIT (2003) and an M.S. and Ph.D. in Applied Physics from Harvard University (2009). He is currently an Associate Professor of Materials Science and Engineering at KAUST. His research group focuses on the study of hybrid organic−inorganic nanoparticles and materials for applications in optoelectronics and catalysis.

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ACKNOWLEDGMENTS This work was supported by King Abdullah University of Science and Technology (KAUST).

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