Direct self-focusing synthesis of monodisperse [Au8(PPh3)7]2+ nanoclusters

Article abstract of DOI:10.1039/c7dt02657a

Direct and scalable synthesis of monodisperse gold nanoclusters is highly desired but remains a great challenge due to the complexity of chemical reactions. In this regard, a suitable precursor is important as it can simplify the synthesis processes and offer controllability in tuning the product. In this study, we found that Au(PPh₃)₂Cl could be used as an effective precursor for the direct synthesis of atomically monodisperse [Au₈(PPh₃)₇]²⁺ nanoclusters without the need of tedious post-synthetic purification steps. The Au(PPh₃)₂Cl precursor could be directly reduced by NaBH₄ (0.25 molar equivalent) in a dichloromethane solution; this produced Au₈ clusters with a 35% reaction yield. Time-dependent mass spectrometry and in situ UV-vis absorption spectroscopy reveal that the synthesis process is initiated by the rapid formation of the Au₆-Au₈ mixture, followed by a slow spontaneous self-focusing process that converges the mixture into atomically monodisperse Au₈. The success of this direct synthesis has been hypothesized to arise from the relatively stronger Au(i)?Au(i) aurophilic attraction between Au(i)-PPh₃ complexes that facilitates the aggregation of Au(i)-PPh₃ on Au(0) cores.

Full text of DOI:10.1039/c7dt02657a

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DOI: 10.1039/C7DT02657A
Direct self-focusing synthesis of monodisperse [Au8(PPh3)7]2+
nanoclusters
Ting Huang,† Li Huang,† Yong Jiang, Fengchun Hu, Zhihu Sun,* Guoqiang Pan, and
Shiqiang Wei
National Synchrotron Radiation Laboratory, University of Science and Technology of
China, Hefei 230029, P. R. China
Abstract: Direct and scalable synthesis of monodisperse gold nanoclusters is highly
desired but remains a great challenge due to the complexity of chemical reactions.
Toward this goal, a suitable precursor is important as it can simplify the synthesis
processes and offer controllability in tuning the product. In this work, we find that
Au(PPh3)2Cl could be used as an effective precursor for the direct synthesis of atomically
monodisperse [Au8(PPh3)7]2+ nanoclusters, without the need of tedious postꢀsynthetic
purification steps. The Au(PPh3)2Cl precursor could be directly reduced by NaBH4 (0.25
molar equivalent) in dichloromethane solution, producing Au8 clusters with a 35%
reaction yield. Timeꢀdependent mass spectrometry and inꢀsitu UVꢀvis absorption spectra
reveal that the synthesis process is initiated by the rapid formation of Au6−Au8 mixture,
followed by a slow spontaneous selfꢀfocusing process that converges the mixture into
atomically monodisperse Au8. The success of this direct synthesis is hypothesized to arise
from the relatively stronger Au(I)···Au(I) aurophilic attraction between Au(I)ꢀPPh3
complexes that facilitates the aggregation of Au(I)ꢀPPh3 on Au(0) cores.
† These authors contributed equally to this work.
* Corresponding authors. Eꢀmail: zhsun@ustc.edu.cn.

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Introduction
Atomicallyꢀprecise gold nanoclusters have acted as a category of nanomaterials and
attracted wide scientific attention, due not only to their unique structures and properties
intermediate between those of the atomic and bulk states, but also to their potential in
developing novel optical nanomaterials1ꢀ6 and catalysts.7ꢀ13 The scalable synthesis of Au
nanoclusters with sizeꢀdependent chemical and physical properties could promote their
fundamental science and applications, and has been an active subject of research in the
last decade. To obtain atomically monodisperse Au nanoclusters in a controllable way,
tremendous efforts have been made and great progresses have been achieved.4, 14ꢀ19
However, to date it is still highly desired but remains great challenge to develop more
facile and effective routes for synthesis of Au nanoclusters.
So far, numerous synthetic routes of Au nanoclusters have been exploited. They could
be grouped into two categories: direct synthetic route and postꢀsynthetic route.15 The
direct synthesis refers to reduction of the precursor complexes (most commonly,
AuPPh3Cl and HAuCl4) by a certain reducing agent to yield atomically monodisperse
nanoclusters. Staring from the pioneering work of the BrustꢀSchiffrin method,20 the direct
synthetic route has been used for preparation of various phosphineꢀ and thiolꢀstabilized
Au nanoclusters via the borohydride or borane reduction of gold complexes in
homogeneous solutions. It is successful in the synthesis of stable Au9/Au11, Au13, and
Au25 nanoclusters during the reduction processes of monoꢀ, diꢀphosphine, or thiolate Au(I)
complexes.21ꢀ25 However, the direct reduction route is unsuccessful in preparing many
other coreꢀsized Au nanoclusters due to the complexity of the chemical reaction systems
that produce a mixture of nanoclusters with various sizes; especially, direct synthesis of
ultrasmall gold clusters consisting of several Au atoms (Au4, Au6, Au7, and Au8) is still
difficult. To solve this problem, many postꢀsynthetic methods have been exploited which
can be regarded as a coreꢀsize conversion or transformation process, where preformed
larger coreꢀsized nanoclusters are converted into smaller, singleꢀsized molecular

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nanoclusters. Typical examples of postꢀsynthetic methods are the synthesis of Au25
nanorods using thiolateꢀinduced sizeꢀconversion of polydisperse Auꢀtriphenylphosphine
(PPh3) nanoparticles (1−3.5 nm),26, 27 and HClꢀetching induced synthesis of diꢀphosphine
protected Au13 clusters from polydisperse clusters.28ꢀ30 Although ultrasmall gold
nanoclusters (Au4, Au6, Au7, and Au8) can be synthesized through postꢀsynthetic routes,18,
22, 31ꢀ34
they suffer from several drawbacks, such as complicated synthesis process, low
reaction yield, and limited choice of inductive agents. Ultrasmall gold clusters with Au
atoms ranging from 6 to 13 are commonly coordinated by phosphine ligands, and form an
important family of molecular nanoclusters. Therefore, seeking more pathways for facile
and effective synthesis of Auꢀphosphine nanoclusters has been a longꢀstanding objective
of cluster science and deserves further study.
It has been known that precursor is of great importance in nanocluster synthesis, at
least in two aspects that a good precursor can simplify the synthesis processes, and can
offer more controllability in targeting the desired nanoclusters and tuning their property.14
Hence, choosing a suited precursor may be a critical factor in a rational design of the
synthetic chemistry of nanoclusters. In this work, we show that reducing the Au(PPh3)2Cl
precursor by NaBH4 (0.25 molar equivalent) could yield atomically monodisperse
[Au8(PPh3)7]2+ nanoclusters, which were obtained previously with complicated
postꢀsynthetic route via multiꢀstep conversion from [Au9(PPh3)8]3+ to [Au8(PPh3)8]2+, and
then [Au8(PPh3)8]2+ to [Au8(PPh3)7]2+.35 The reaction yield of this synthesis is 35% on the
basis of Au atom and could be easily scaled up for largeꢀquantity synthesis.
Timeꢀdependent mass spectrometry and UVꢀvis absorption spectra were used to trace this
synthesis and revealed the twoꢀstep processes, including the first kinetic reduction and
the second selfꢀfocusing steps. Similar reaction starting from the commonly used
AuPPh3Cl precursor yields mainly [Au11(PPh3)8Cl2]3+ but no sign of [Au8(PPh3)7]2+,
highlighting the critical role of the Au(PPh3)2Cl precursor in this synthesis.

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Experimental
Chemicals: Chloroauric acid (HAuCl4, Alfa Aesar), triphenylphosphine (99%, Alfa
Aesar), sodium borohydride (98%, Aldrich). All solvents were obtained from Aldrich and
employed as received. The Au(PPh3)2Cl and Au(PPh3)2Cl precursors were prepared
according to the protocols in the literature.36
The synthesis of [Au8(PPh3)7]2+ clusters: NaBH4 (0.0562 mmol) was added to the
Au(PPh3)2Cl precursor (20 mmol/L, 10 mL of CH2Cl2) solution, under rapid stirring at 25
°C for 168 h. The final solution was precipitated after addition of 200 mL of nꢀhexane for
24 h. The supernatant was discarded and the precipitates obtained through centrifugation
were redissolved in 2 mL of ethanol. After the solution was centrifuged at 15000 rpm for
5 min to remove insoluble Au(I) complexes, the supernatant containing [Au8(PPh3)7]2+
clusters was evaporated to dryness by vacuum rotary. The obtained solid was
reꢀdissolved in ethanol and collected by centrifugation/evaporation again. This step was
repeated for 2 times, giving the yield of ~ 35% [Au8(PPh3)7]2+ cluster on the basis of Au
atom.
The effect of the reaction temperature on the synthesis was also tested. It was found
that within a narrow temperature range from 25 to 40 °C, the formation process of the
end product could not be accelerated remarkably with increase of temperature. However,
further elevating the reaction temperature to beyond 40 °C led to the growth into much
larger clusters or even nanoparticles, instead of monodisperse [Au8(PPh3)7]2+ clusters.
The reaction starting from the AuPPh3Cl instead of Au(PPh3)2Cl as precursor: It
was conducted under the otherwise same reaction conditions as those using the
Au(PPh3)2Cl precursor. And all its characterizations were the same with those in the case
of Au(PPh3)2Cl.
To two asꢀprepared Au8 cluster solutions (1.2 µmol/L, 10 mL of CH2Cl2),
Au(PPh3)2Cl and AuPPh3Cl precursor with the same mole of 0.05 mmol was added, and

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then the reaction solution was stirred rapidly at 20 °C for 168 h. Finally, the obtained
solutions were characterized by mass spectrometry and UVꢀvis absorption spectra.
Characterizations: Electrospray ionization mass spectrometry (ESIꢀMS) analysis
was carried out by ESIꢀTOF with a Bruker Microtof massꢀspectrometer operating in
positive ionization mode. The ionization process needed Temperature (100 °C) control
and nitrogen drying gas (1 ꢀL/min) in ESI source. All spectra were obtained in reflection
mode of the TOF mass spectrometer equipped with multiꢀstep detection for maximum
sensitivity; isotopic resolution was obvious throughout the entire detection mass range of
m/z 800 to 2000. Calibration was conducted by reserpine (609 m/z) in FIA 10 pg (100:1
S/N 200 ꢀL/min). The detection of clusters was carried out with 10 ꢀL of sample at flow
rate of 0.1 ml/min in 0.1% formic acid in acetonitrile/H O (1:1). The Au L3ꢀedge Xꢀray
2
absorption nearꢀedge structure (XAFS) spectra were obtained in transmission mode at the
1W1B beamline of Beijing Synchrotron Radiation Facility (BSRF), China, with electron
storage ring operated at 2.5 GeV and a maximum current of 250 mA. The UVꢀvis
absorption data were collected by a Purkinje General TUꢀ9001 spectrometer in the
wavelength range of 250–800 nm, following the correction of the background absorption
by utilizing pure ethanol. Xꢀray photoelectron spectroscopy (XPS) measurements were
carried out by using a VG Thermo ESCALAB 250 spectrometer operated at 120 W with
the binding energy calibrated against the carbon 1s line.
Results and Discussions
The reaction procedure for the direct oneꢀpot synthesis of [Au8(PPh3)7]2+ nanoclusters
is shown schematically in Fig. 1. It involves the reduction of Au(PPh3)2Cl to produce
polydisperse Au6−Au8 intermediates, and their subsequent selfꢀfocusing toward
monodisperse Au8 clusters. Upon the addition of NaBH4 to the Au(PPh3)2Cl precursor
solution, the reduction occurs rapidly in 1 h, producing a redꢀbrown solution. Then the
solution color undergoes nonꢀobvious change till the completion of the reaction (168 h).

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After repeated centrifugation/evaporation for the reaction product, the synthetic reaction
yields ~35% [Au8(PPh3)7]2+ clusters on the basis of Au atom. To the best of our
knowledge, this is the first time for direct synthesis of monodisperse [Au8(PPh3)7]2+
nanoclusters through using the Au(PPh3)2Cl precursor. Previously, [Au8(PPh3)7]2+
nanoclusters could only be obtained via multiꢀstep postꢀsynthetic conversion from
[Au9(PPh3)8]3+ to [Au8(PPh3)8]2+, and then [Au8(PPh3)8]2+ to [Au8(PPh3)7]2+.35
Figure 1. Direct synthesis of Au8 nanoclusters via the selfꢀfocusing process during
NaBH4 reduction of Au(PPh3)2Cl.
Next, timeꢀdependent ESIꢀMS and in-situ UVꢀvis absorption spectra were used to
identify the intermediate species and monitor their evolution during the direct synthesis
of [Au8(PPh3)7]2+. Aliquots of the solution were taken at regular time intervals and
subject to characterizations. Seen from the ESI mass spectra shown in Fig. 2(a), over the
range of m/z 900ꢀ2000 Da, the precursor prior to the reaction (0 h) exhibits a strong peak
at m/z 919 Da corresponding to [Au2(PPh3)2]+. After reduction for 1 h, besides weak
signals over the range of m/z 900ꢀ1200 assigned to Au(I) complexes (mainly Au2 and Au3
complexes), distinct peak at m/z 1706 Da appears, which could be assigned to the
molecular formula of [Au8(PPh3)7]2+ (calculated m/z = 1706.19 Da). Other weaker peaks
at m/z 1377 and 1608 Da are also observable, and they are related to [Au6(PPh3)6]2+
(calculated m/z = 1378.10 Da) and [Au7(PPh3)7]2+ (calculated m/z = 1608.42 Da),
respectively. The determinations of these molecular formulas are confirmed by the good
match between the experimental data and the calculated isotope patterns Fig. 2(b). This

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indicates that the reduction products of Au(PPh3)2Cl at 1 h are mainly composed of
polydisperse Au6−Au8 cluster mixture. With increasing reaction time, the Au8 signal is
gradually enhanced, and the Au6 and Au7 signals are gradually damped. Finally, at 168 h,
one intense peak corresponding to Au8 is observed in the mass spectrum, while the peaks
of Au6 and Au7 clusters are nearly invisible, implying the atomical monodispersity of the
Au8 product.
(a)
(b)
(c)
Experimental
Calculated
0 h
[Au8(PPh3)7]2+
[Au8(PPh3)7]2+
1706
1708
1 h
0 h
[Au7(PPh3)7]2+
[Au7(PPh3)7]2+
370
412
460
9 h
[Au6(PPh3)6]2+
1608
1610
1 h
9 h
[Au6(PPh3)6]2+
48 h
48 h
1378
1380
168 h
168 h
400
600
1200
1800
Wavelength (nm)
m/z
m/z
Figure 2. (a) Timeꢀdependent ESIꢀMS of Au8 synthetic process via the direct reduction of
Au(PPh3)2Cl precursor. (b) Comparison between the experimental ESI mass spectra and
the calculated isotope patterns of [Au8(PPh3)7]2+, [Au7(PPh3)7]2+ and [Au6(PPh3)6]2+. (c)
Timeꢀdependent UVꢀvis absorption spectra of the reduction process as in (a).
The findings of the ESI mass spectrometry could be confirmed by the
timeꢀdependent UVꢀvis absorption spectra (Fig. 2(c)). A featureless monotonous decrease
is observed in the absorption spectrum of the Au(PPh3)2Cl precursor (0 h). Then
absorption peaks at 412 and 460 nm, characteristic of [Au8(PPh3)7]2+ clusters, start to
appear after 1 h of reduction reaction. Compared with the standard spectrum of pure
[Au8(PPh3)7]2+ cluster that exhibit three distinct absorption peaks at 370, 412 and 460
nm,18 at this time the 370 nm absorbance is absent and the 460 nm absorbance is too
weak in intensity, suggesting the polydispersity of the clusters. With prolonged reaction

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time, the absorption peaks become more and more distinct; at 48 h the absorbance at 370
nm appears. Finally, the absorption spectrum shows a wellꢀdefined feature of
[Au8(PPh3)7]2+ clusters, indicative of the obtainment of monodisperse [Au8(PPh3)7]2+
clusters evolved from mixed clusters. Since in this later stage of the synthesis, no
additional reagent was added to converge the sizes of the intermediate clusters, the
spontaneous sizeꢀconvergence in the Au(PPh3)2Cl circumstance could be called a
“selfꢀfocusing” process.
Figure 3. (a) Timeꢀdependent ESIꢀMS of the direct reduction process of AuPPh3Cl
precursor by NaBH4. (b) Comparison between the experimental ESI mass spectra and the
calculated isotope patterns of [Au2(PPh3)2Cl]+, Au2(PPh3)3Cl]+, [Au3(PPh3)2Cl2+H+]+,
[Au9(PPh3)8]3+, [Au9(PPh3)8]2+ and [Au11(PPh3)8Cl2+2H+]3+. (c) Timeꢀdependent UVꢀvis
absorption spectra of the reduction process as in (a).
The combined analyses of ESIꢀMS and UVꢀvis absorption spectra manifest that the
Au(PPh3)2Cl precursor is beneficial to the oneꢀpot synthesis of monodisperse
[Au8(PPh3)7]2+ clusters. One may wonder what happens if the commonly used Au(I)
precursor of AuPPh3Cl precursor is employed instead. To clarify this issue, we performed
parallel experiments by using the AuPPh3Cl precursor to replace Au(PPh3)2Cl under the
otherwise same reaction conditions. The reaction process was also monitored by ESIꢀMS

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and UVꢀvis absorption spectra as shown in Fig. 3. Compared with the Au(PPh3)2Cl case,
the mass spectrum of AuPPh3Cl reduction product at 1 h indicates a wider size
distribution. Apart from the [Au6(PPh3)6]2+ (m/z = 1377 Da) and [Au7(PPh3)7]2+ (m/z =
1608 Da) peaks, other intense peaks at m/z~ 1290, 1446, and 1936 Da are apparent which
could be assigned to the molecular formulas of [Au9(PPh3)8]3+ (calculated m/z = 1290.08
Da), [Au11(PPh3)8Cl2+2H+]3+ (calculated m/z = 1446.11 Da), and [Au9(PPh3)8]2+
(calculated m/z = 1936.52 Da), respectively, as evidenced by the excellent match between
experimental and calculated isotope patterns (Fig. 3(b)). The major mass peaks are the
ones of [Au9(PPh3)8]2+, [Au9(PPh3)8]3+ and [Au6(PPh3)6]2+. As the reaction continues to
10 and 22 h, the Au11 mass peak is gradually enhanced. After 168 h of reaction time, the
dominant cluster is [Au11(PPh3)8Cl2+2H+]3+, accompanied by the [Au6(PPh3)6]2+ and
[Au9(PPh3)8]3+ species. It should be noted that almost no [Au8(PPh3)7]2+ peak could be
observed throughout the synthesis course. This is in good agreement with the literature
report that NaBH4 reduction of AuPPh3Cl is the traditional way to synthesize Au11 and
Au9 clusters.18, 25 The eventual formation of [Au11(PPh3)8Cl2]3+ and [Au9(PPh3)8]3+ is also
indicated by the timeꢀdependent UVꢀvis absorption spectra (Fig. 3(c)), which well
resembles those of the standard spectrum of Au11 clusters in both the spectral shape and
peak positions (312, 380, 416, and 500 nm).18, 25, 31 Again, no sign of [Au8(PPh3)7]2+
could be observed. All these observations indicate that [Au8(PPh3)7]2+ could be hardly
obtained through the direct reduction of the AuPPh3Cl precursor.
Summarizing the above results, the stable cluster of [Au8(PPh3)7]2+ is formed
through the direct reduction of Au(PPh3)2Cl by NaBH4 (0.25 molar equiv), while the
same reaction starting from the AuPPh3Cl precursor produces [Au11(PPh3)8Cl2]+ clusters.
Because the only changed parameter in these two synthesis reactions is the precursor, the
results suggest that the Au(PPh3)2Cl precursor is vital to the direct synthesis of
[Au8(PPh3)7]2+. In order to shed some light on the correlation between the precursors and
cluster products, the electronic features of Au atoms were investigated by Au L3ꢀedge

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Xꢀray absorption nearꢀedge structure (XANES) spectra and Xꢀray photoelectron
spectroscopy (XPS). The normalized XANES spectra of the Au(PPh3)2Cl and AuPPh3Cl
precursors show that the former exhibits a much more intense whiteꢀline peak at ~11925
eV than that of the latter (Fig. 4 (a)). This indicates the more electronic transitions from
the 2p3/2 to the 5d5/2, 3/2 states, resulting in higher Au valence state in Au(PPh3)2Cl.
Similar phenomenon could also be observed from the XPS spectra in Fig. 4 (b). The Au
4f7/2 peak of Au(PPh3)2Cl is located at 85.6 eV, showing a 0.3 eV shift toward the higher
binding energy side as compared with the peak (85.3 eV) of AuPPh3Cl. Intriguingly,
analogous difference persists in the Au valence states of their reduction products. The
[Au8(PPh3)7]2+ cluster exhibits stronger whiteꢀline intensity and higher absorption edge
position than [Au11(PPh3)8Cl2]+ (Fig. 4 (c)), and the Au 4f7/2 XPS peak of [Au8(PPh3)7]2+
is also ~0.6 eV higher (85.3 vs 84.7 eV) than that of [Au11(PPh3)8Cl2]+ (Fig. 4 (d)). We
also performed the Au L3ꢀedge XANES calculations for [Au8(PPh3)7]2+ and
[Au11(PPh3)8Cl2]+ clusters using the FEFF8.0 code37 based on their known structures. The
results are shown in Figure 4(c), which indicates that with decreasing cluster size from
[Au11(PPh3)8Cl2]+ to [Au8(PPh3)7]2+, the whiteꢀline is intensified due to stronger Au→P
charge transfer, in agreement with the experimental observations. Hence, the Au valence
state in [Au8(PPh3)7]2+ is remarkably higher than that in [Au11(PPh3)8Cl2]+, just like their
corresponding precursors.

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1.2
0.8
0.4
(a)
(b)
AuPPh3Cl
Au(PPh3)2Cl
Au 4f
1.2
85.34
85.6
AuPPh3Cl
Au(PPh3)2Cl
0.8
0.4
0.0
0.0
1.5
11920
11960
E (eV)
12000
84
86
88
90
92
Binding Energy (eV)
[Au11(PPh3)8Cl2]+
(c)
(d)
[Au8(PPh3)7]2+
1.2
0.8
0.4
0.0
84.65
85.25
1.0
0.5
0.0
[Au11(PPh3)8Cl2]+
Au 4f
[Au8(PPh3)7]2+
Au11 calculation
Au8 calculation
84
86
88
90
92
11920 11940 11960 11980
E (eV)
Binding Energy (eV)
Figure 4. (a)/(c) XANES, and (b)/(d) XPS spectra of the precursors/clusters. The dashed
lines in (c) show the calculated XANES spectra for [Au8(PPh3)7]2+and [Au11(PPh3)8Cl2]+
clusters.
As for why Au(PPh3)2Cl and AuPPh3Cl give quite different cluster products under the
same reduction conditions, we hypothesize that this is related to the different valence
states of Au atoms in the precursors. Figure 5 illustrates schematically the formation
process of [Au8(PPh3)7]2+. After addition of the reducing agent NaBH4, zerovalent Au
cores are produced and initiate the growth of clusters. However, owing to the insufficient
reducing agent (merely 0.25 molar equiv), a significant part of precursors remains
unreduced and releases into the solution Au(I)ꢀPPh3 complexes that acts as reactive
species during the subsequent cluster growth. According to the “grow and passivate”
model,14 these active Au(I)ꢀPPh3 complexes progressively aggregate on the freshly
generated Au(0) cores and passivate the structure until a stable [Au8(PPh3)7]2+ cluster
configuration is achieved. The impetus for the aggregation growth of the clusters is
provided by the direct Au(I)−Au(0) bonding in the clusters that results in the short
Au−Au distance (2.63−2.72 Å, average 2.68 Å) in [Au8(PPh3)7]2+,35 as well as the

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Au(I)···Au(I) aurophilic attraction between the Au(I) complexes.38 The aurophilic
interactions, whose strength is in the order of tens of kJ·mol−1 and comparable with
hydrogen bonds, arise from the hybridization between the empty 6s/6p shells and filled
5d10 shells due to the relativistic effect.39 As a result of the weaker Au(I)···Au(I)
aurophilic attraction than the direct Au(I)−Au(0) binding, the Au(I)−Au(I) distances
35
(2.71−2.94 Å) with an average value of 2.85 Å,
is longer than the Au(I)−Au(0)
distances. Therefore, the formation of the target Au nanoclusters relies on the
accessibility of the Au(I)ꢀPPh3 complexes to the Au(0) cores and the Au(I)···Au(I)
aurophilic attraction strength as well, which are affected by the valence of the precursor
complexes. The higher Au valence state in Au(PPh3)2Cl strengthens the Au(I)···Au(I)
aurophilic attraction and facilitates the aggregation of the Au(I)ꢀPPh3 complexes around
the Au(0) cores, leading to the formation of thermodynamically stable cluster
[Au8(PPh3)7]2+ with a higher Au valence. As a comparison, the AuPPh3Cl precursor has a
relatively lower Au valence, and hence the aurophilic attraction between the detached
Au(I) complexes is relatively weaker. Consequently, [Au11(PPh3)8Cl2]+ clusters are
finally formed due to the relatively weaker aurophilic attraction, as suggested by the
considerably longer Au(I)−Au(I) distances (in the range of 2.86−3.13 Å, 2.95 Å in
average)40 than those in [Au8(PPh3)7]2+.
Figure 5. Schematic illustration of the aggregation growth mode of [Au8(PPh3)7]2+
reduced from Au(PPh3)2Cl. The Au(I)···Au(I) aurophilic attraction between Au(I)ꢀPPh3
complexes provides impetus for the stabilization of the clusters.

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Conclusions
In conclusion, we report a direct oneꢀpot synthetic protocol for monodisperse
[Au8(PPh3)7]2+ cluster through reducing the Au(PPh3)2Cl precursor by NaBH4. Our
synthesis is facile and simple, and could be used for largeꢀscale synthesis of
monodisperse Au8 clusters. Timeꢀdependent ESIꢀMS and UVꢀvis absorption spectra
reveal the twoꢀstep processes of this synthesis. The first step is the rapid formation of the
Au8ꢀdominated cluster mixture, and the second step involves a slow selfꢀfocusing process
in which the cluster mixture grows into monodisperse Au8 cluster in excess Au(PPh3)2Cl.
The comparative study by using the AuPPh3Cl precursor shows that Au11, rather than Au8,
is eventually formed, reflecting the critical role of Au(PPh3)2Cl in the oneꢀpot direct
synthesis of the Au8 clusters. The success of this direct Au8 synthesis possibly arises from
the relatively stronger Au(I)···Au(I) aurophilic attraction between Au(I) complexes
detached from Au(PPh3)2Cl molecules than from AuPPh3Cl.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant No.
11475176, U1632263, and 21533007), and the Foundation for Innovative Research
Groups of the National Natural Science Foundation of China (11621063). The authors are
thankful to BSRF for the synchrotron radiation beamtime.
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Table of Contents Entry
Direct synthesis of atomically monodisperse Au8 nanoclusters via the self-focusing
process during NaBH4 reduction of Au(PPh3)2Cl.