Fabrication of thiol-capped Pd nanoparticles: An electrochemical method

Article abstract of DOI:10.1063/1.1562334

The fabrication of thiol-capped Pd nanoparticles by an electrochemical method was discussed. The structure, bonding and electronic properties of the electrodeposited Pd nanoparticles (NP) were studied. The analysis showed that the thiol-capped electrodeposits were metallic Pd particles of a few nanometers with local structure and electronic behavior different from the non-thiol-capped electrodeposits.

Full text of DOI:10.1063/1.1562334

Fabrication of thiol-capped Pd nanoparticles: An electrochemical method
P. Zhang and T. K. Sham
Citation: Applied Physics Letters 82, 1778 (2003); doi: 10.1063/1.1562334
View online: http://dx.doi.org/10.1063/1.1562334
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APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 11
17 MARCH 2003
Fabrication of thiol-capped Pd nanoparticles: An electrochemical method
P. Zhang and T. K. Shama)
Department of Chemistry, University of Western Ontario, London, Ontario, Canada, N6A 5B7
͑
Received 5 November 2002; accepted 14 January 2003͒
A simple electrochemical method is developed to prepare thiol-capped Pd nanoparticles on a Si
100͒ surface by reducing Pd2 in solution in the presence of thiol molecules. The structure,
ϩ
͑
bonding, and electronic properties of the electrodeposited Pd nanoparticles ͑NPs͒, together with a
series of Pd model systems, were studied by electron microscope and x-ray absorption spectroscopy
at the S K-edge and the Pd L -edge. The thiol-capped electrodeposits are found to be metallic Pd
3
,2
particles of a few nanometers, with local structures and electronic behavior considerably different
from the non-thiol-capped electrodeposits, but rather comparable to colloidal thiol-capped NPs.
©
2003 American Institute of Physics. ͓DOI: 10.1063/1.1562334͔
In the past several years, metal nanoparticles ͑NPs͒ sta-
bilized by capping molecules, such as thiols, have received
considerable attention due to their interesting optical and
electronic properties and potential applications.1 Many ef-
forts have been dedicated to developing efficient routes to
Pd NPs of ϳ2.4 nm synthesized by the colloidal method,11
CDPdS11; and ͑3͒ commercially available palladium sulfide,
PdS ͑99.9%, Aldrich͒. Room-temperature x-ray absorption
experiments at the S K-edge and the Pd L -edge were per-
formed at the CSRF-DCM beamline at the Synchrotron Ra-
diation Center ͑SRC͒, University of Wisconsin-Madison.
InSb ͑111͒ crystals were used to provide monochromatized x
rays with an energy resolution E/⌬E of 4000 ͑ϳ0.6 eV at
the S K-edge͒. The monochromator was calibrated with Pd
foil. The XANES spectra were background removed and
normalized.12
A dramatic difference between the deposits synthesized
with and without thiols is visibly apparent by inspection. The
EDPd sample appears as a relatively rugged brown film on
the Si surface, while EDPdS11 appears as a uniform blue
film. The FESEM images ͑Fig. 1͒ further reveal their mor-
phological difference. As seen in Figs. 1͑a͒ and 1͑b͒, par-
ticles of several tens of nanometers with a large size distri-
bution are seen in the EDPd film. However, Figs. 1͑c͒ and
1͑d͒ reveal that the EDPdS11 film consists of particles of a
few nanometers. This observation immediately indicates that
the presence of thiol additives not only reduces the size of
the electrochemically produced Pd, but significantly im-
,2
3
,2
3
synthesize a variety of thiol-capped metal NPs such as Au,
4
5
6
7
Pd, Ag, Cu, Ir, and so on. However, nearly all the meth-
ods that have been reported so far are based on the colloidal
chemistry strategy, involving the reduction of the metal salt
7
with reducing agents such as NaBH and ‘‘superhydride’’
4
followed by capping with thiols. The electrodeposition
method ͑often known as electroplating͒, has been known for
a long time as a mild-conditioned, fast, and easily-controlled
route to reduce metal salt into elemental metals. This method
has been successfully applied in recent years in the synthesis
8
of various metal nanostructures such as nanowires,
9
10
nanorods, and nanoparticles. Here, we report a simple
electrochemical route to the synthesis of thiol-capped Pd
NPs on a silicon surface: In the presence of thiols, Pd2 from
ϩ
PdNO in methanol is electroreduced on the surface of a
3
silicon wafer into metallic Pd, which is then capped by the
thiols. The existence of the thiol-capped Pd NPs was con-
firmed by field-emission scanning electron microscopy
͑FESEM͒ and x-ray absorption near-edge structure ͑XANES͒
at the S K-edge and Pd L -edge. From the XANES results,
3
,2
the correlation between the electronic behavior and local
structure of the nanodeposits emerges.
The synthesis was conducted in a nitrogen-protected,
two-electrode cell, where a HF-refreshed, n-type Si͑100͒
Ϫ1
͑
ϳ5–10 ⍀ cm ͒ was used as the working electrode and a Pt
foil as the counter. By applying a negative constant current
2
2ϩ
Ϫ3
͑
0.5 mA/cm ͒ for 3 min, Pd from a 10 -M PdNO solu-
3
Ϫ3
tion was reduced into Pd metal in the presence of a 10 -M
dodecanethiol solution ͑in methanol͒. The sample, hence-
forth denoted EDPdS11, was then rinsed with methanol for
several times before being dried in the air. A series of rel-
evant Pd samples were also studied for comparison. They
are: ͑1͒ electrodeposited Pd in the absence of thiols ͑same
current density, PdNO concentration, and deposition dura-
3
tion as that for EDPdS11͒, denoted EDPd; ͑2͒ thiol-capped
FIG. 1. FESEM images of ͑a͒, ͑b͒ Pd particles ͑EDPd͒ electrodeposited in
the absence of thiols and ͑c͒, ͑d͒ Pd particles ͑EDPdS11͒ electrodeposited in
the presence of thiols. The images were taken using a Hitachi S-4500
FESEM operated at 5.0 kV.
a͒Electronic mail: sham@uwo.ca
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On: Sat, 29 Nov 2014 12:27:23
0

Appl. Phys. Lett., Vol. 82, No. 11, 17 March 2003
P. Zhang and T. K. Sham
1779
FIG. 3. The palladium L3,2-edge XANES spectra of thiol-capped colloidal
nanoparticles ͑CDPdS11͒, thiol-capped Pd electrodeposits ͑EDPdS11͒, non-
thiol-capped electrodeposits ͑EdPd͒ and Pd foil.
FIG. 2. The sulfur K-edge XANES spectra of thiol-capped electrodeposited
Pd NPs ͑EDPdS11͒, thiol-capped colloidal Pd NPs ͑CDPdS11͒ together with
free thiols ͑RSH͒ and PdS. For the free thiols, HOOC–(CH2)10–SH mol-
ecules ͑solid, Aldrich͒ were used instead of dodecanethiols ͑liquid͒ for the
convenience of XANES measurement in a vacuum system.
Pd foil in Fig. 3, EDPdS11 XANES shows a noticeable
broadening and damping in all the resonance peaks, indicat-
ing a large static disorder resulting from reduced outer shell
backscattering amplitude ͑Pd-Pd͒ in the thiol-capped NPs
and the presence of Pd–S contribution. This is in agreement
with the observation in Fig. 1, in that there are fewer outer
shells on average in nanoparticles.
proves the uniformity of the deposits as well. Close exami-
nation of the FESEM images reveals that in the absence of
thiol, the electrochemical procedure produces large Pd par-
ticles covering less than half the substrate surface ͓Fig. 1͑a͔͒.
In the presence of thiol, however, almost all the surface area
of the Si substrate is covered by very closely packed Pd
nanoparticles ͓Fig. 1͑c͔͒. Thus, the presence of thiols inhibits
the further growth of the Pd nuclei, resulting in the formation
of smaller nuclei that aggregate to form a more closely
packed nanoparticle film.
Figure 4 shows the threshold region of the L -edge
3
XANES of the Pd samples. The sharp resonance just above
the threshold is called the white line and is associated with
Pd 2p3/2-to-4d transition ͑the larger the d-hole count, the
more intense the white line͒. The intensity of the white line is
proportional to the unoccupied densities of states of the Pd
15
d-band above the Fermi level. From Fig. 4, several inter-
esting observations are noted. First, the L -edge absorption
3
Figure 2 shows the S K-edge XANES of the EDPdS11
and CDPdS11, together with that of free thiol and PdS. The
resonance at the threshold of the RSH and PdS is 1s-to-
threshold of EDPdS11 shifts by 0.7 eV to higher energy rela-
tive to the Pd foil and EDPd, both of which are nearly iden-
tical. Second, there is a considerable increase in the white-
line intensity of EDPdS11 and CDPdS11 relative to the Pd
␴
*-type transition ͑projected unoccupied p densities of states
of S character͒ associated with S–C and S–Pd interaction,
respectively. The resonance peaks a and b of EDPdS11 are
comparable to those of CDPdS11, both significantly different
from that of free thiol, which shows only a S–C peak. Peak
a in the two NP samples are comparable to the S–Pd reso-
nance of PdS. They are attributed to the S–Pd bond. Peak b
is attributed to the S–C resonance, which is shifted to higher
energy relative to that of the free thiol due to the formation
of sulfur–metal bond. A similar peak has been observed in
foil. It should be noted that the L -edge XANES of
3
EDPdS11 is almost the same as that of CDPdS11. They are
much less intense than that of PdS, a nominally Pd͑II͒ com-
pound. Finally, in the post edge region, EDPdS11 shows a
broad structure that is comparable to that of PdS in energy
position. The enhanced whiteline of EDPdS11 relative to
bulk Pd ͑the foil͒ indicates a d-charge depletion at the Pd site
in the NPs. This is also in agreement with the shift of its
absorption threshold. Since 4d charge screens the core better
than 5s or 5p, a d→sp rehybridization would yield a posi-
1
3
thiol-capped Au NPs. A closer inspection of the XANES of
EDPdS11 and CDPdS11 reveals that both peaks of EDPdS11
are narrower than those of CDPdS11, and that while peak a
of Pd-S resonance shifts to slightly higher energy, peak b of
the S–C resonance shifts to lower energy. These differences
are indicative of the slightly different local structure between
the palladium–sulfur interaction on the corresponding NP
surface. This can be associated with a slightly shorter Pd–S
bond in CDPdS11 in terms of multiple-scattering
1
4
considerations. It is conceivable that stronger Pd–S inter-
action takes place on the surface of the smaller colloidal NPs
͑ϳ2.4 nm͒, pushing the S–C resonance to higher energy
above the threshold ͑a shorter Pd–S bond͒.
Figure 3 shows the Pd L -edge XANES above the
3
,2
threshold of the two deposits and a Pd foil. The similar os-
cillating patterns in the L -edge region, that is, peaks c and d,
3
are the extended x-ray absorption fine structure signature of
FIG. 4. XANES spectra near the Pd L -edge white-line region of PdS,
CDPdS11, EDPdS11, EDPd, and Pd foil.
3
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1
5
fcc metallic Pd. Compared with the XANES of EDPd and
On: Sat, 29 Nov 2014 12:27:23

1780
Appl. Phys. Lett., Vol. 82, No. 11, 17 March 2003
P. Zhang and T. K. Sham
tive threshold shift. Using the established method1 and the
d-hole count of Pd metal, 1.78,16 we calculated from the
white-line intensity the d-hole count of EDPdS11 and
CDPdS11 to be 1.96, ϳ10% increase relative to the bulk.
The origin of decreased d-charge count in thiol-capped
nanoparticles is interesting. It has been known that naked ͑or
capped with weakly interacting molecules such as dendrim-
ers͒ metal NPs, such as Pd and Au, tend to gain d-charge
6,17
SRC is supported by US NSF grant DMR-00-84402 and
CSRF by NSERC and NRC of Canada. The research at
UWO is supported by NSERC of Canada. The authors are
grateful to G. Kim for collecting some of the Pd L -edge
3
XANES and Dr. Y. F. Hu, Dr. A. Jurgensen and Dr. K. Tan
for technical assistance at CSRF-SRC. One of the authors
͑P.Z.͒ is grateful to the support from Ontario Graduate Schol-
arship and R. R. Lumsden Fellowship for his Ph.D. research.
relative to the bulk metal, that is, showing a less intense
L3-edge white line.1
7,18
This is not too surprising since sur-
face atoms in Au are known to gain d-charge relative to bulk
1
1
9
M. Brust, M. Walker, D. Bethell, D. Schiffrin, and R. Whyman, R. , J.
Chem. Soc. Chem. Commun. 1994, 801 ͑1994͒.
A. Templeton, W. Wuelfing, and R. W. Murray, Acc. Chem. Res. 33, 27
͑2000͒.
Au atoms. Therefore, the contribution of a nanosize effect
to the depletion of d-charge in EDPdS11 must be counterbal-
anced by the surface chemistry, which induces charge flow
from the NP to the sulfur (Pd→S) via the surface Pd–S
interaction, since S is more electronegative ͑2.58͒ than Pd
2
3
D. V. Leff, P. C. Ohara, J. R. Heath, and W. M. Gelbart, J. Phys. Chem 99,
7036 ͑1995͒; R. L. Whetten, J. T. Khoury, M. M. Alvarez, S. Murthy, I.
Wezmar, Z. L. Wang, P. W. Stephen, C. L. Cleverland, W. D. Luedtke, and
U. Landman, Adv. Mater. 5, 428 ͑1996͒; A. C. Templeton, S. Chen, S. M.
Gross, and R. W. Murray, Langmuir 15, 66 ͑1999͒.
S. Chen, K. Huang, and J. A. Stearn, Chem. Mater. 12, 540 ͑2000͒; I.
Quiros, M. Yamada, K. Kubo, J. Mizutani, M. Kurihara, and H. Nishihara,
Langmuir 18, 1413 ͑2002͒.
͑
2.20͒. Similar surface charge transfer effect was also found
2
0
17
theoretically and experimentally in the case of thiol-
capped Au NPs. The first resonance peak following the white
line of Pd arises from multiple scattering and has been
known to be sensitive to the local environment of Pd
4
5
6
7
S. Y. Kang and K. Kim, Langmuir 14, 226 ͑1998͒.
1
6,18
atoms.
In the case of EDPdS11, the first resonance after
S. Chen and J. M. Sommers, J. Phys. Chem. B 105, 8816 ͑2001͒.
C. K. Yee, R. Jordan, A. Ulman, H. White, A. King, M. Rafailovich, and
J. Sokolov, Langmuir 15, 3486 ͑1999͒.
C. R. Martin, Science 266, 1961 ͑1994͒.
Y. Y. Yu, S. S. Chang, C. L. Lee, and C. R. C. Wang, J. Phys. Chem. B
101, 6661 ͑1997͒.
M. T. Reetz and W. Helbig, J. Am. Chem. Soc. 116, 7401 ͑1994͒; R. M.
Penner, J. Phys. Chem. B 106, 3339 ͑2002͒.
The synthesis of colloidal thiol-capped Pd NPs was carried out following
the method in Ref. 4 and the size of the obtained NPs were determined by
transmission electron microscopy and x-ray diffraction.
T. Ressler, J. Synchrotron Radiat. 5, 118 ͑1998͒.
P. Zhang and T. K. Sham ͑unpublished͒.
J. Stohr, NEXAFS Spectroscopy ͑Springer, Berlin, 1992͒; T. K. Sham, Acc.
Chem. Res. 19, 99 ͑1986͒.
the white line is an indication of the increased contribution
of the attachment of thiolate species on the surface of Pd NPs
as the size of the NPs becomes smaller, in agreement with
the observation in Fig. 2. Thus, this observation indicates
that the nanocrystallite size in EDPdS11 approaches that of
CDPd11 ͑2.4 nm͒ as supported by the identical Pd white
lines of EdPdS11 and CdPdS11. From the FESEM image in
Fig. 1͑d͒, the size of the NPs is found to be ϳ10 nm. Thus it
is very plausible that these ϳ10-nm particles consist of still
smaller nanocrystallines.
8
9
10
11
1
1
2
3
14
To summarize, we have reported an electrochemical
method to prepare reasonably uniform Pd NPs on a silicon
surface in the presence of thiol. The existence of
alkanethiolate-capped Pd NPs was confirmed by FESEM and
both the S K- and Pd L -edge XANES. From the XANES
1
1
5
6
T. K. Sham, Phys. Rev. B 13, 1903 ͑1985͒; I. Coulthard, D. T. Jiang, J. W.
Lorimer, and T. K. Sham, Langmuir 9, 3443 ͑1993͒.
I. Coulthard and T. K. Sham, Phys. Rev. Lett. 77, 4824 ͑1996͒.
P. Zhang and T. K. Sham, Appl. Phys. Lett. 81, 736 ͑2002͒.
K. Ohtani, T. Fujikawa, T. Kubota, K. Asakura, and Y. Iwasawa, Jpn. J.
Appl. Phys. 37, 4134 ͑1998͒.
17
1
1
8
9
3
,2
results, it was also found that the 4d-charge at the Pd sites in
the electrodeposited NPs on average depletes by ϳ10% rela-
tive to bulk Pd metal. This observation is attributed to the
interplay of surface and quantum size effects in small NPs.
P. H. Citrin, G. K. Wertheim, and Y. Baer, Phys. Rev. Lett. 41, 1425
͑
1978͒.
20
H. Hakkinen, R. Barnett, and U. Landman, Phys. Rev. Lett. 82, 3264
͑1999͒.
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