Characterization of the unstability of 4-mercaptoaniline capped platinum nanoparticles solution by...

Characterization of the unstability of 4-mercaptoaniline capped

platinum nanoparticles solution by combining LB technique

and X-ray photoelectron spectroscopy

Frederic Raynal a, Arnaud Etcheberry a, Sara Cavaliere a,b,Vincent Noel b, Henri Perez b,*

a Institut Lavoisier (IREM, UMR 8637 CNRS), Universite de Versailles-Saint Quentin, 45 ave des Etats Unis, 78035 Versailles, Franceb CEA-Saclay, DSM/DRECAM/SPAM-LFP, Bat. 522, 91191 Gif-sur-Yvette, France

Received 26 April 2005; received in revised form 24 May 2005; accepted 25 May 2005

Available online 1 July 2005

Abstract

This paper reports on the study of the evolution of 4-mercaptoaniline ( p-HSC6H4NH2) functionalized platinum nanoparticles

in solution by coupling the Langmuir–Blodgett technique and X-ray photoelectron spectroscopy (XPS). The spectra are

recorded on mixed LB films containing fatty acid and platinum particles in proportion 50/50. Several samples built from fresh

and aged solutions of particles are analyzed. Comparison of the Pt 4f, S 2p and N 1s regions in each case points to the time

dependant chemical evolution of the functionalized particles involving at once platinum, thiolate and amine components. The

particle aging in solution is reported during several months, until the complete flocculation of the functionalized platinum

nanoparticles. Using the compared XPS analysis of the LB layers obtained from the different particle solutions, unstability of the

storage looks then clearly related to the chemical evolution of the bifunctional organic crown.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Platinum nanoparticles; Functionalization; Langmuir–Blodgett technique; XPS

www.elsevier.com/locate/apsusc

Applied Surface Science 252 (2006) 2422–2431

1. Introduction

Self-assembled monolayers (SAMs) formed on

metals provide an elegant route to the preparation of

well-defined organic assemblies on solid surfaces.

These SAMs consist essentially in thiolate derivatives

* Corresponding author. Tel.: +33 1 69 08 41 83;

fax: +33 1 69 08 12 13.

E-mail address: [email protected] (H. Perez).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2005.05.042

adsorbed on flat surfaces of gold [1–3], silver [1],

copper [1], iron [4], nickel [5] or platinum [6].

Recently, an electrochemical way has been described

to form SAMs on flat surfaces of gold [7].

The strong interaction involving sulfur and metal

atoms has been exploited to control the formation of

noble metal nanoparticles of gold [8–13], platinum

[14–20], silver [14,21], or gold/silver alloy [22]. An

efficient approach in the elaboration of nanoparticle

thin film is the Langmuir–Blodgett (LB) technique

.

F. Raynal et al. / Applied Surface Science 252 (2006) 2422–2431 2423

[23–26]. In a previous study, we described the

elaboration of LB thin films of 4-mercaptoaniline

( p-HSC6H4NH2) functionalized platinum nanoparti-

cles [25].

The aging of the structures involving self-

assembled thiolate monolayer is presumably related

to the stability of the chemical bond between thiolate

and metal surfaces. Oxidation of thiolate to sulfonate

species has been evidenced on flat surfaces of silver,

copper, as well as gold, and related to air oxidation

processes [1,2,27]. In a previous paper, we reported

similar degradation of 4-mercaptoaniline functiona-

lized platinum nanoparticles in air atmosphere. The

degradation was evidenced by infrared spectroscopy

recorded on the powder stored in air several months

after the synthesis [16]. The spectrum exhibits new

absorption attributed to sulfonates species. Moreover,

we also pointed out the slow aggregation process of

the nanoparticles in DMSO solution. This process was

suspected to be closely related to the chemical

evolution of the 4-mercaptoaniline capping molecules.

In the current work, we characterize the step by step

chemical evolution of the organic crown of the particle

stored in solution by combining Langmuir–Blodgett

technique and X-ray photoelectron spectroscopy

(XPS). Solutions prepared just after the synthesis of

the nanoparticles are referred as fresh solutions

whereas solutions which have been kept in flask for

at least ten days are referred as aged solutions. To

follow the aging process aliquots of an initially fresh

solution are taken and used to build up LB films,

which are then analyzed by XPS.

2. Experimental

All products were obtained from Aldrich and were

used as received without further purification.

2.1. Synthesis

Platinum nanoparticles capped with mercaptoani-

line molecules were prepared by reduction of PtCl4 in

hexylamine by sodium borohydride. Just after the

reduction 4-mercaptoaniline is added to the reaction

media resulting in the functionalisation of the

platinum nanoparticles [16]. This procedure provides

particles with an average diameter of 2 nm that can be

dissolved in dimethyl sulfoxide (DMSO). The solu-

tions are prepared in sealed flask that are stored under

air until further utilization.

2.2. LB films elaboration

The formation of Langmuir–Blodgett (LB) thin

films of 4-mercaptoaniline functionalized nanoparti-

cles has been already described [25]. The films are

transferred on glass slides provided with gold

electrodes deposit using a laboratory-made LB trough

filled with Millipore-grade water (resistivity higher

than 18 MV cm). The transfer is carried out vertically

at a surface pressure of 28 mN/m with a transfer speed

of 0.3–0.5 cm/min. The substrate being hydrophilic an

odd number of monolayers can be deposited. In this

work the samples consist in three monolayers thick LB

films containing behenic acid (CH3-(CH2)20-COOH)

and platinum nanoparticles. The proportion of fatty

acid to nanoparticles is defined by the ratio of the area

associated respectively to both species at the air water-

interface which is set at 50/50 [25]. In the three

monolayers configuration, the Au 4f signal provided

by the buried interface, can be detected and used as an

internal reference.

2.3. XPS characterization

XPS analysis is performed using a VG Esca-

lab_220i XL system. High-resolution XPS conditions

have been fixed, i.e. constant analyzer energy mode

with 20 eV as pass energy, and a monochromatic Al

Ka X-ray excitation giving high-resolution spectra.

The analyzed area is 1 mm2 and the contact is taken on

the inter-digital gold electrodes part where none LB

deposition is present. Binding energies (BE) of all core

levels are referenced to the Au 4f 7/2 core level

(BE = 84 eV). Curve fitting is performed using the

Eclipse data system developed by VG Scientific.

3. Results

3.1. XPS spectra of samples built from fresh

nanoparticle solutions

The XPS spectra are recorded first on freshly

synthesized particles from which a fresh DSMO

F. Raynal et al. / Applied Surface Science 252 (2006) 2422–24312424

solution is prepared. The period of time between the

isolation of the powder, the preparation of the DMSO

solution and the LB film elaboration is approximately

two hours. The LB sample is then analyzed within

twelve hours after elaboration.

Spectrum analysis is performed on several core

levels: N 1s, S 2p, O 1s, C 1s, Pt 4f and Au 4f that are

characteristic of the particles deposited on substrate

provided with gold electrodes. The most interesting

feature is observed for S 2p region, which is shown in

Fig. 1. Several contributions are distributed between

161 and 171 eV. The main contribution at low binding

energy exhibits an apparent spin orbit splitting related

to the 2p character of the core level. For comparison

the S 2p region of a CdS oxide free layer [28], is also

proposed in Fig. 1 with the same XPS resolution as for

the functionalized particles. It appears that this

reference S 2p region presents an only one contribu-

tion with a perfectly defined spin orbit splitting.

According to the S 2p region of a CdS sample, easily

Fig. 1. S 2p region of sulfur in freshly 4-mercaptoaniline functio-

nalized platinum nanoparticles compared to S 2p region of sulfur in

a CdS structure [28].

fitted with one split contribution [28], the modified

shape of the S 2p region of functionalized nanoparti-

cules at low BE cannot be fitted, in its mean feature,

with only one contribution, but with at least two. So,

for the nanoparticles the S 2p peak fitting must be

considered with attention. It is presented in Fig. 2. The

S 2p fitting parameters are deduced from the ones used

for the CdS signals for which we have taken into

account only the effect of the spin-orbit splitting that

gives rise to a doublet. Three specific parameters of the

2p doublet have been fixed: the branching ratio 3/2:1/2

equal to 2, the distance between S 2p3/2 and S 2p1/2

taken equal to 1.2 eV, and the FWHM equal to 1.2 eV

for each sub level. These parameters are then injected

to fit the mean feature of the signal of nanoparticles.

Clearly a double contribution is detected on the lowest

binding energy region. For the S 2p contributions at

higher BE, where the 2p3/2 and 2p1/2 sub levels cannot

be obviously differentiated a simple component must

be used. Finally, with a rather good reproducibility,

a four component structure can be always proposed

for describing the S 2p spectra associated to the

nanoparticles associated to fresh solution.

The other core levels can be easily fitted, with one

(Pt 4f, Au 4f, N 1s) or two (C 1s, O 1s) contributions.

The Pt 4f fitting is also given in Fig. 2. The only

parameter fixed for the Pt 4f doublet, fitted in

asymmetric mode, is the branching ratio 7/2:5/2 equal

to 1.3.

Finally fitted BE positions and FWHMs for the

different components of each region investigated are

reported in Table 1. We note that all the FWHM are

Table 1

Binding energy (BE) and full width at half maximum (FWHM) of

the core levels investigated in LB films built using freshly 4-

mercaptoaniline functionalized platinum nanoparticles

Region BEa (eV) FWHM (eV)

C 1s 285.1 1.1

284.5 1.1

O 1s 532.6 1.9

531.2 1.5

N 1s 399.5 1.6

Pt 4f7/2 71.3 1

S 2p3/2 162.3 1.2b

162.7 1.2b

S 2p 164.9 2.6

167.7 2.5a Values are determined with respect to the Au 4f7/2 BE at 84 eV.b Fixed value in the fit.


Fig. 2. Fitted S 2p and Pt 4f XPS spectra of LB films elaborated from freshly 4-mercaptoaniline functionalized platinum nanoparticles. Arrows

in the S 2p region show the S 2p3/2 and S 2p1/2 contributions at lower energy.

included between 1 and 1.9 eV except for the S 2p

contributions at higher BE, where the 2p3/2 and 2p1/2

sub levels cannot be differentiated.

3.2. LB films built from aged particles solutions

The LB thin films were elaborated from aged

nanoparticle solutions of respectively 3 weeks and 3

months. The corresponding XPS spectra were

recorded in each case. The BE shifts of the XPS

signals are reported in Table 2. Compared to those of

Table 2

Binding energy (BE) and relative peak area of N 1s, Pt 4f7/2, S 2p3/2 and S

mercaptoaniline functionalized platinum nanoparticles

Sample Fresh solution Three wee

BE (eV) Peak area (%) BE (eV)

S 2p3/2 162.3 38 162.3

162.7 40 162.8

S 2p 164.9 16 165.6

167.7 6 167.8

Pt 4f7/2 71.3 100 71.4

N 1s 399.5 100 399.5

400.3

Values are determined with respect to the Au 4f7/2 BE at 84 eV.

the fresh sample, the Pt 4f, S 2p and N 1s regions of the

sample prepared from 3 weeks aged solution exhibit

more or less important evolution. The general

tendency is a peak shift to higher energy which is

tiny for Pt 4f, more pronounced for S 2p and quite

important for N 1s. The latter shows a clear

broadening.

The Pt 4f, S 2p and N 1s regions of the sample built

from 3 months aged solution are presented in Fig. 4.

Whereas N 1s region shows similar features than after

3 weeks the changes become clearly marked for S 2p

2p regions investigated in LB films built using freshly and aged 4-

ks aged solution Three months aged solution

Peak area (%) BE (eV) Peak area (%)

21 162.3 14

50 163.2 60

15 166.2 13

14 168.2 13

100 71.4 44

72.3 33

73.2 23

51 399.4 46

49 400.5 54


and Pt 4f. The shift of the S 2p signal to higher

BE is increasing and the Pt 4f region is characterized

by the emergence of a new contribution at higher

BE.

Several modifications are therefore observed with

samples built from aged particles solutions. The peak

fitting allows to precise these evolutions which are

reported in Table 2. It contains the BE energies and the

relative area of the peaks when several BE are

identified for a given element.

4. Discussion

4.1. XPS spectra of samples built from fresh

nanoparticle solutions

The comments concern essentially the S 2p region,

which exhibits four different contributions. The

main signal below 165 eV was fitted with two

S 2p contributions, related to sulfur in two different

chemical environments. The S 2p3/2 at low BE, located

at 162.3 � 0.05 eV, is in perfect agreement with

results from 4-mercaptoaniline functionalized gold

nanoparticles reported by Johnson et al. (BE of S 2p3/2

at 162.5 eV and BE of Au 4f7/2 at 84.2 eV) [10].

Data from literature for SAMs on metal flat surfaces

[1,2,5,6,29,30], and more recently on organically

capped nanoparticles [10,19,21,22,31,32], indicate that

S 2p3/2 at low BE (BE = 162.3 � 0.05 eV) results

from the dissociative adsorption of the S–H bond (or

S–S bond when disulfide instead of thiol is involved) on

the metal (Eq. (1)):

RSH þ Mð0Þ ! RS-MðIÞ þ 1=2H2 (1)

The S 2p3/2 BE is related to the charge density on the

sulfur atom, as was demonstrated in the well known

work of Siegbahn and co-workers [33]. This work,

based on 132 organic sulfur compounds ranging from

thiolates to sulfonium ions, presented a linear free

energy relationship (LFER) between the S 2p3/2 BE

and the calculated charge of the sulfur. In our case,

interpolation of S 2p3/2 into the Siegbahn LFER

suggests that the sulfur bears a charge of (�0.2e),

which is consistent with a thiolate like Pt–S bond with

covalent character.

A second S 2p3/2 contribution is located at

162.7 � 0.05 eV. Interpolation of S 2p3/2 into the

Siegbahn LFER suggests that this sulfur bears a charge

of (�0.1e). It is likely that such a BE does not involve

oxidized sulfur. Considering that Pt nanoparticles have

a cuboctahedral shape [25] and taking into account the

small size of the core (ca. 2 nm) the fraction of

platinum atoms that belongs to Pt(1 0 0) faces is quite

low compared to Pt(1 1 1) faces [34]. Therefore, the

two sulfur contributions cannot be related to Pt–S

bonds corresponding to different platinum faces

because both sulfurs are found in equal quantities

(Table 2). The attribution of this second S 2p structure

to free alkylthiols or disulfides cannot be completely

excluded because the expected binding energies are

�163.2 eV [6,17,29,30,33]. However, the fact that the

nanoparticles undergoes several washing after synth-

esis conflicts with such a conclusion. A third point that

could explain two thiolate like peaks regards the

binding modes which can be expected for a thiolate

species on a given crystal face. Indeed, theoretical

studies on alkanethiolates suggested that there were

different binding sites for thiolates on Au(1 1 1)

surfaces referred as top, bridge and hollow sites

[35]. The relative strength of the Au–S bond in the

various sites was in the order hollow > bridge > top.

Although quite difficult to prove such different

binding sites could be tentatively proposed to be

responsible for the two different environments

corresponding to Pt–S bonds at 162.3 and 162.7 eV.

The S 2p peak fitting shows also the presence of

two sulfurs exhibiting BE higher than 164 eV. The

following attributions can be done in agreement with

literature: sulfinate and sulfonate species respectively

at 164.9 � 0.05 and 167.7 � 0.05 eV. These struc-

tures correspond therefore to oxidized species of 4-

mercaptoaniline, consistent with previous observa-

tions [16]. In contrast to other para substituted

arylthiolate, 4-mercaptoaniline looks actually easily

oxidizable, as suggested by similar XPS results

recorded with 4-mercaptoaniline functionalized gold

nanoparticles [10].

Even if the small size of the nanoparticle makes the

surface atoms a significant part of the total atoms

number, numerous data reported in literature

[10,19,21,22,31,32] do not give clear evidence on

the possibility to distinguish between the metal

surface atoms which are combined to the capping

molecule, and those from the internal part of the

particle. With that respect, our results are consistent


with most of those reported in literature. The Pt 4f7/2

signal of fresh sample, reported in Fig. 1, is located at

71.2 � 0.05 eV, a value consistent with platinum

nanoparticles in the metallic Pt(0) state. The shape and

FMWH of the Pt 4f7/2 peak (Table 1) do not allow to

distinguish between different platinum environments.

The N 1s region of fresh sample, reported in Fig. 1, is

located at 399.5 � 0.05 eV, a value consistent with a

primary amine function. As we previously shown [25],

this point confirms that there is no interaction between

the carboxylic function of the fatty acid, which is used

to build up LB films containing nanoparticles, and the

amine function of the latter. At least for freshly

prepared solution the question of a competitive

binding between amine and thiolate function with

respect to platinum surface can be reasonably ruled

out for two reasons. First, the synthesis proceeds in

hexylamine as temporary stabilizing agent before

disulfide introduction [16], if disulfide is omitted then

the reaction media flocculates irreversibly suggesting

that the interaction with amine function with platinum

is weaker than with thiolate. Second, if the interaction

of the amine function with platinum was efficient then

the nanoparticles would be crosslinked through amine

and thiolate links and could not be solubilized

spontaneously.

4.2. LB films built from 3 weeks aged particles

solution

The S 2p region of LB films elaborated from 3

weeks aged solution is presented in Fig. 3. At first

view this S 2p region looks rather similar to the fresh

sample ones. However, peak fitting and relative peak

area point to a real evolution characterized by an

increasing proportion of sulfur S 2p at higher binding

energies compared to the lowest BE of 162.3 eV. This

evolution suggests a possible reorganization of Pt–S

bonds for which the component at 163.8 eV became

majoritary with respect to one at 162.3 eV. Possibly

related to this evolution, the same tendency is

observed for S 2p signals at BE higher than 164 eV.

S 2p located at 167.8 eV appears now with an

equivalent relative weight than the one pointing at

lower energy (165.6 eV).

Besides the evolution observed on the S 2p signals,

the N 1s contribution exhibits a strong evolution. The

peak, given in Fig. 3, cannot be fitted anymore with a

unique contribution attributed to primary amine

function (BE N 1s at 399.5 � 0.05 eV), but a second

contribution centered at 400.3 eV has to be introduced

and is found to be in an equivalent proportion

(Table 2). The BE of the second N 1s contribution

looks quite low to reflect an oxydized or quaternized

species [33]. It has to be mentionned that a double

structure of N 1s peak is reported in 4-aminothio-

phenol monolayers on gold in pristine and oxidized

form [36]. In this paper a complex oxidation

mechanism of 4-mercaptoaniline is proposed that

finally results in 40-mercapto-N-phenyl quinone. It

cannot be excluded that such a process occurs on

platinum nanoparticles. In contrast to sulfur and

nitrogen evolution the Pt 4f peak barely evolves except

for a very small displacement (0.1 eV) towards higher

BE.

4.3. LB films built from 3 months aged particles

solution

In Fig. 4, we report the S 2p region for samples built

from 3 months aged solution. The evolution in term of

binding energy shift is now obvious. The evolution

observed after 3 weeks looks more pronounced after 3

months, the Pt–S bond at lower energy (162.3 eV)

appears to decrease in favour of a new peak that points

at 163.2 eV (Table 2). This BE could be now attributed

to disulfide species [6,17,29,30,33]. The S 2p signals

attributed to oxydized sulfur are located at higher

energies (166.2 and 168.2 eV) but their relative

intensities do not increase in a significant way. This

point could be related to the elimination of oxydized

species (the interaction of which with platinum

surface is supposed to be weak) during the LB

procedure. However, this explanation is not conforted

by a significant evolution of the Pt/S ratio between the

different samples showing the quite complex pro-

cesses taking place in the chemical evolution reported

here. Whereas the N 1s signal does not exhibit

important evolution compared to those observed on

samples built from 3 weeks aged solution the Pt 4f

signal is characterized by a strong modification. The

signal cannot be fitted with one contribution but

requires three contributions located at 71.4, 72.3 and

73.2 eV. The lowest energy contribution is still

attributed to Pt(0) whereas the new ones correspond

to Ptd+ forms of platinum. Referring to the literature,


Fig. 3. XPS spectra of LB films elaborated 3 weeks after 4-mercaptoaniline functionalized platinum nanoparticle synthesis; (- - -) fresh

particles; (—) 3 weeks after particle synthesis.

the Pt 4f7/2 BE at 73.2 � 0.05 eV is characteristic of

PtO whereas the Pt 4f7/2 BE at 72.3 � 0.05 eV can be

attributed to PtOads and/or Pt(OH)2 [37,38]. A possible

explanation of this evolution could be related to the

fact that the surface is not passivated anymore because

of the pronounced degradation of the initial capping

4-mercaptoaniline.

4.4. XPS of particles after one year

In a previous paper, we described LB films

elaboration using 4-mercaptoaniline functionalized

platinum nanoparticles, as well as their electrical

properties. We pointed out the fact that within 3

months, the area per particle calculated at the air-water


Fig. 4. XPS spectra of LB films elaborated 3 months after 4-mercaptoaniline functionalized platinum nanoparticle synthesis; (- - -) fresh

particles; (—) 3 months after particle synthesis.

interface decreases by ca. 50% [25]. This behavior is

now clearly related to the chemical degradation of the

organic crown surrounding the particles than we report

here. The ultimate step of this process is the

macroscopic flocculation of the solution.

In a last set of experiment, we recorded the XPS

spectra of the platinum precipitate resulting from the

complete flocculation of a one year old solution. This

precipitate was briefly washed with acetone and the

cast powder was then analyzed (Fig. 5). On this

sample, about 80% of the sulfur detected is into

sulfinate and sulfonate forms, whereas only 30% of the

sulfur was oxidized in the sample built from 3 months

aged solution.

The peak fitting of the Pt 4f region shows the same

contributions already observed after 3 months with

one difference: the Pt 4f7/2 contribution at higher

BE is shifted at 74.4 � 0.05 eV instead of 73.2 �0.05 eV. This Pt 4f signal can be attributed to PtO2,

involving a platinum evolution after several months in

DMSO from PtO to PtO2. The XPS quantitative

analysis gives a Pt(0)/Ptd+ ratio close to 36/64, close

to the sample built 3 months after particle synthesis

(Pt(0)/Ptd+ = 44/56).


Fig. 5. XPS spectra of the precipitate collected in a 1-year aged solution of 4-mercaptoaniline functionalized platinum.

Along this paper we focussed on the spectroscopic

study to characterize the functionalized nanoparticle

and its chemical evolution. In spite of the good

reproducibility of the shape and BE of the XPS regions

analyzed, the correlation between these chemical

evolutions and the quantitative analysis gave surpris-

ing results. Except for a consistent general tendency

showing an increase of the oxygen contents in samples

from aged solution, we noticed that the various atomic

ratio where moving into a large range from one sample

to another. This point contrasts with the consistent

quantitative analysis recently we reported on such LB

films bearing increasing number of layers and built

from fresh solutions [39].

5. Conclusion

X-ray photoelectron spectroscopy measurements

have been conducted on Langmuir–Blodgett films of

4-mercaptoaniline functionalized platinum nanopar-

ticles built from fresh and aged solutions. For fresh

solutions the S 2p region binding energy confirms the

expected formation of a thiolate-like bond between


sulfur and platinum. Reasonable fitting implies the

introduction of two different thiolates binding sites

corresponding to different environments.

We have pointed out the step-by-step degradation

of the organic crown in DMSO solution (storage

solution) and found a good correlation with XPS

spectroscopic characterization. This process looks

quite complex and seems to involve reorganization of

Pt–S bonds and sulfur environment. These observa-

tions look related to the chemical stability of 4-

mercaptoaniline itself, which is oxydizable and

explain the time unstability of the solution of

nanoparticles. Further publication will report on the

stabilization of the solution of platinum nanoparticles

by chemical reaction of the amine function of 4-

mercaptoaniline crown.

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