Short Articles

Ligand Eﬀect on the Formation ofGold Nanoparticles via Sputtering
Deposition over a Liquid Matrix
Ikumi Akita, Yohei Ishida,and Tetsu Yonezawa*
Division of Material Science andEngineering, Faculty of Engineering,
Hokkaido University, Kita 13 Nishi 8,
Kita-ku, Sapporo, Hokkaido 060-8628

T. Yonezawa

E-mail: tetsu@eng.hokudai.ac.jp
Received: May 24, 2016; Accepted: June 7, 2016;
Web Released: June 14, 2016

Abstract

Sputtering deposition of gold over a liquid matrix containing diﬀerent ligands with thiolate, amine, or carboxy groupsresulted in the formation of spherical gold nanoparticles (AuNPs) in the range of 2.1 to 6.7 nm, and the diameters gradually decreased as the aﬃnity of the ligands for Au increases.

Sputtering deposition is a typical physical method for preparing thin ﬁlms on solid substrates.1,2 Such physical deposition methods for nanomaterials can be applied for the preparation of nanoparticles.3­5 In a general sputtering process, ionizedgas (argon or nitrogen) at high voltage attacks the target metaland ejects metal atoms or clusters, which fall onto a substrateand form a ﬁlm. All electrically conductive materials can beused, and the properties of the resultant ﬁlms can be easily controlled by varying the mechanical parameters. Thus, this facile
method is well established and is widely used on the industrial
scale.
A recently developed method involving sputtering overliquid matrices has attracted considerable attention as a novel
preparation technique of colloidal metal nanoparticles (NPs).
Ejected atoms and clusters coalesce into NPs in the gas phaseor at the surface of the liquid matrix. The NPs could furthercoalesce into larger particles in the absence of stabilizing
reagents. For this purpose, ionic liquids with no vapor pressure
have been used in the early stages of such works,6 and the useof other non-volatile liquids such as liquid monomers7­9 andpolymers10,11 has also begun. This sputtering method is promising as a novel and environmentally friendly alternative forpreparing metal NPs without the use of reductants.12,13 These
investigations have enabled the synthesis of metal NPs (mostlyAu NPs) with diameters of one to several tens of nanometers.
Recently, we developed a novel methodology, called thematrix sputtering method, to systematically control the size andphotophysical properties of metal NPs obtained by sputtering
process. While previous approaches have relied on the viscosity of the liquid matrix for controlling particle diameter,10 ournew approach, taking inspiration from chemical methods,
introduced thiolate ligands as a stabilizer in the liquid matrix.
1054 | Bull. Chem. Soc. Jpn. 2016, 89, 1054–1056 | doi:10.1246/bcsj.20160187

The thiolate can dissolve in the liquid matrix14­16 or evaporatein the gas phase,11 and can eﬀectively coordinate with the NP
surfaces, in accordance with their concentrations, to preventthe aggregation and growth of NPs. This allows the systematic
size control of NPs in a single nanometer order. We havealready reported several examples for size-controlled synthesisof Au or Ag NPs using neutral,9,11,14 anionic,15 or cationic16
thiolate ligands.
In a typical chemical method, thiolate, amine, and carboxylic
acid are mainly used as monomeric stabilizing ligands.13 The
diﬀerence in their aﬃnity for the NP surface results in diﬀering
particle diameter controllability and photophysical properties.
In the past, our matrix sputtering system has focused on thiolate
ligands because they have the highest aﬃnity for Au, and thiswork aims to investigate the eﬀect of ligand structure with
diﬀerent functional groups in a sputtering system. While theeﬀect of ligand structure has been well established in the chemical reduction method, this work presents its eﬀect on a physical(sputtering) synthetic method, where the mechanism of nanoparticle formation is very diﬀerent from that of the chemical
method, which is through reduction of metal precursors.
Three kinds of short ligands with the same chain length anddiﬀerent functional groups, viz. 6-mercapt-1-hexanol (6MH),
6-amine-1-hexanol (6AH), and 1-heptanoic acid (HA) wereused in this study, as summarized in Figure 1. The bonding
strength of each functional group to Au is estimated to beapproximately 45, 8, and 2 kcal mol¹1 for thiolate, amine, andcarboxylic acid, respectively.17 The α,α¤-diglycerol (DG) wasemployed as the liquid matrix, as our previous work hasrevealed that DG could be used for this purpose due to its very
high boiling point (bp = 412 °C at 760 mmHg).13
Four types of sputtering systems were studied in this work:(a) DG without organic ligands, (b) 0.5 M HA in DG, (c) 0.5 M
6AH in DG, and (d) 0.5 M 6MH in DG. After sputtering
deposition under the same conditions except for the liquidmatrix (see detailed procedure outlined in Supporting Information), extinction spectra of Au NP dispersions were measured in a quartz cell with 1 mm optical path (Figure 2). Clear
absorption peaks were observed at 530 nm for samples (a) and(b), and at 509 nm for sample (c). These wavelength regions
are attributed to the localized surface plasmon absorption of Au
NPs (see the red color of the obtained suspension (Figure S1)).
Surface plasmon absorption generally arises from coherent
oscillations of free electrons on the surface of NPs in response
to a speciﬁc wavelength. Therefore, plasmon absorption isbased on the diameter of a given particle.18,19 Thus, the red shift
from 509 to 530 nm in Figures 2a­2c may indicate that 6AHmodiﬁed Au NPs (Figure 2c) have a smaller diameter than theother NPs (Figures 2a and 2b). On the other hand, sample (d)did not show any plasmon absorption peaks but a new absorp-Figure 1. Chemical structures of (a) DG, (b) HA, (c) 6AH,and (d) 6MH.

© 2016 The Chemical Society of Japan

Figure 2. Extinction spectra of Au NPs prepared by sputtering over (a) DG, (b) HA in DG, (c) 6AH in DG, and(d) 6MH in DG. The spectra were measured in a 1 mmquartz cell.

tion shoulder was observed at around 350 nm. Typically, Au
NPs with a diameter of less than 2 nm prepared and isolated
by chemical methods do not show plasmon absorption.20 The
spectral feature observed in Figure 2d strongly indicates thatthe obtained 6MH-modiﬁed Au NPs are already nanoclusters
with size-dependent HOMO­LUMO absorption characteristics.
Note that the peak at 350 nm for sample (c) comes from 6AH
itself (Figure S2).
Figure 3 shows TEM images and size-distribution histograms of samples (a­d). Coalesced particles were observed
for samples (a) and (b), while well-passivated spherical particles were found in samples (c) and (d). The average diametersof NPs in samples (a­d) were determined to be 6.7 « 3.2,4.6 « 0.9, 3.0 « 0.8, and 2.1 « 0.8 nm, respectively. Sincenanoparticle formation in sample (a) depends only on the viscosity of DG, the aggregation of NPs is not suppressed. Thus,relatively larger NPs were formed in sample (a). The obtained
average diameter (6.7 nm) is similar to that of Au NPs preparedin other kinds of liquid matrices, such as ionic liquids or poly(ethylene glycol).6,10,11 The average particle diameter of sample (b) was slightly smaller than that of sample (a); however,most particles were in a similar size-distribution range (2­6nm). This is attributed to the weak passivation by HA. Indeed,sample (b) showed cross-linking aggregation of NPs that isprobably due to the weak passivation by carboxy groups of theligand. The absorption spectral changes in the plasmonic peaksin Figure 2 in samples (a), (b) (both at 530 nm), and (c) (509nm) are attributable to the coalescence of NPs, wherein sample (c) maintains the particle shape due to eﬀective passivationby amine groups while samples (a) and (b) easily coalesce intolarger particles due to the relatively weak passivation by carboxy groups. Judging from these results, it is obvious that thestabilizing ligand with higher aﬃnity to Au produces smaller
NPs. This tendency can be understood from the order of thebonding strength of thiolate, amine, and carboxylic groups toAu (44, 8, and 2 kcal mol¹1, respectively17). 6MH with athiolate group could rapidly passivate the NP surface, and thus
the sputtered NPs did not further coalesce into larger plasmonic
particles. These results clearly suggest that the diameter of NPs
produced via physical (sputtering) process could be controlled
Bull. Chem. Soc. Jpn. 2016, 89, 1054–1056 | doi:10.1246/bcsj.20160187

Figure 3. TEM images and size-distribution histograms ofAu NPs prepared by sputtering over (a) DG, (b) HA in DG,(c) 6AH in DG, and (d) 6MH in DG. The histograms wereprepared from 100 NPs counted in more than three TEM
images.

by varying the kinds of functional groups of the stabilizing
ligand, similar to the typical chemical method.21,22 Note thatthe size of metal NPs that reach the surface of a liquid matrix
or solid surface by sputtering is around 1­2 nm (although theprecise dimension depends on the type of metal, sputtering
current, and the distance between the metal target and liquidsurface).23 Moreover, the decrease in particle size reﬂects thedecrease in plasmon absorption presented in Figure 2: particles
with a diameter larger than 3 nm (samples a­c) showed plasmon
absorption, which was not observed for sample (d).
Since sample (d) did not show plasmonic absorption and a
new absorption shoulder was observed at 350 nm, we studied
its ﬂuorescence property. The other samples (a­c) did not
ﬂuoresce, while sample (d) ﬂuoresced in the near IR region
under excitation at 305 nm. The ﬂuorescence and excitation© 2016 The Chemical Society of Japan | 1055

ture, Sports, Science and Technology (MEXT), Japan. Authors
thank Drs. Y. Wang and M. T. Nguyen for their kind experimental assistance and fruitful discussions.
Supporting Information
Detailed experimental procedure, sample pictures of Au NPs
suspension prepared in 6MH in DG, 6AH in DG, and HA in
DG (Figure S1), and extinction spectra of 6AH in DG before
sputtering deposition (Figure S2). This material is available on
http://dx.doi.org/10.1246/bcsj.20160187.
References
Figure 4. Excitation and ﬂuorescence spectra of Au NPs
prepared by sputtering over 6MH in DG (sample d). The
excitation wavelength was set at 305 nm, and the excitation
spectrum was observed at 707 nm.

spectra are shown in Figure 4. Excitation and ﬂuorescencemaxima were observed at 310 and 707 nm, respectively. This
ﬂuorescence property is quite similar to that reported in ourprevious studies on Au NPs prepared by sputtering method
using several types of thiolate ligands as stabilizer.7,8,11,16
Current Au NPs (2.1 nm diameter) prepared by chemical
reduction typically do not ﬂuoresce in the visible region.20 A
similar phenomenon has been reported for gold sputtering withthiolate ligands, and the obtained Au NPs with diameter in therange of 1.0­3.3 nm showed near IR ﬂuorescence with a small
spectral change according to their TEM diameters.7,8,11,16 It is
well known that the ﬂuorescence wavelength of such verysmall noble metal clusters depends on the number of atoms
forming the cluster. Thus, our results on ﬂuorescent nanoparticles with a much larger diameter are strange, assumingthat the nanoparticles are single crystals. Judging from thedetailed characterization of obtained Au NPs, we theorized in
our previous papers that NPs formed via matrix sputtering are
not single crystals, but instead are formed by the aggregation of very small, multinuclear Au complexes or clusters.11
The mechanism of such unusual ﬂuorescence phenomena will
be reported soon by high-performance liquid chromatographyusing the size-fractionation technique.
In this work, we reported the diﬀerence in the protectivityof several stabilizing monomer ligands with thiolate, amine,and carboxy groups, used in the sputtering process for thepreparation of Au NPs. TEM diameters gradually decreased as
the aﬃnity of the ligands for Au increased, similar to typicalchemical syntheses. To our knowledge, this work is the ﬁrst
systematic investigation of the ligand eﬀect on the diameter ofNPs obtained through a physical synthetic method (matrixsputtering method).
This work was partially supported by a Research Fund from
Hokkaido University (to YI and TY), and by Building ofConsortia for the Development of Human Resources in Scienceand Technology (to YI). TY acknowledges the support from
Murata Foundation. This work was also partially supported by
microstructural characterization platform as a program ofNanotechnology Platform of the Ministry of Education, Cul-1056 | Bull. Chem. Soc. Jpn. 2016, 89, 1054–1056 | doi:10.1246/bcsj.20160187

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© 2016 The Chemical Society of Japan