R. Ahmad et al. / Polymer 57 (2015) 12e20
15
Table 1
monomer and water as the solvent. The glass vessel was degassed
by bubbling argon for 10 min. The slides were then exposed to UV
light at 365 nm at room temperature for 5 h. The slides were rinsed
with copious amounts of water and ethanol to remove the un-
reacted monomers, and dried in a nitrogen stream.
Surface atomic percentages determined by XPS for bare and functionalized Au plates
AueC6H4e[Oe(CH2)2](n¼0-3)-DEDTC obtained after 1 h incubation of the gold sub-
strate with the corresponding diazonium salt.
Materials
Au
C
O
S
N
Bare Au plate
70
42
42
48
44
25
44
43.5
37
39
5
e
e
3
2
2
1.5
AueC6H4eCH2-DEDTC
AueC6H4-[O-(CH2)2]-DEDTC
AueC6H4-[O-(CH2)2]2-DEDTC
AueC6H4-[O-(CH2)2]3-DEDTC
7
10
11
13
4
2.5
2
2.5. Immobilization of gold nanoparticles (AuNPs)
In order to immobilize AuNPs on the aryl-terminated iniferters
grafted on gold surface, the substrates were first placed in a quartz
flask containing 5 ml of 0.01 M aqueous solution of 4-amino-
2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and were
irradiated for 25 min by UV light (365 nm) at room temperature.
The substrates were subsequently rinsed with water and ethanol
and stored under argon. In a second step, they were incubated with
gold nanoparticles suspensions during 12 h, and then washed
several times with water and ethanol.
2.5
component, while the small peak at 288 eV is due to the dithio-
carbamate SeC]S group. It is noteworthy that a shake-up satellite
p p* is observed at ca. 291 eV, which is the characteristic signa-
ꢁ
ture of the presence of aryl groups. Furthermore, the differences in
the oligo(ethylene oxide) spacer size are clearly visible in the C1s
high resolution spectra, with an enhancement of the CeO
component at 286.6 eV when the number of ethylene oxide units
increases in the spacer.
2.6. Instrumentation
The growth of the iniferter layer at the surface of gold was then
investigated by varying the time of incubation between the gold
substrate and the 2NeC6H4-[O-(CH2)2]2-DEDTC iniferter-derived
Photopolymerization was performed using the commercial ul-
traviolet processor Spectrolinker XL 1500 UV (Spectronics Corp.).
This processor was equipped with 6 tubes (8 W) with a wavelength
range of 365 nm and intensity of 17.6 mW cmꢁ2. XPS spectra were
recorded using a Thermo VG Scientific ESCALAB 250 system fitted
þ
diazonium salts.
The gold signal attenuation due to the progressive covering of
the substrate by the iniferter layer as a function of time was used to
provide an estimation of the organic layer thickness evolution. For
this, the relative attenuation of the Au 4f7/2 signal was expressed as:
with
a
microfocused, monochromatic Al
Ka X-ray source
(hn
¼ 1486.6 eV; spot size ¼ 650
mm; power ¼ 15 kV, 200 W). The
pass energy was set at 150 and 40 eV for the survey and the narrow
regions, respectively. Spectral calibration was determined by
setting the main C1s component at 285 eV. The surface composi-
tion was determined using the integrated peak areas and the cor-
responding Scofield sensitivity factors corrected for the analyzer
transmission function. Scanning electronic microscopy (SEM) im-
ages were obtained using a Zeiss SUPRA 40 FESEM equipped with a
thermal field emission gun. Images were created using SMARTSEM.
Wetting properties was characterized with a Contact Angle Digi-
drop PX 500. The contact angle measurement were performed
using the image of a sessile drop at the points of intersection be-
tween the drop contour and the projection of the surface (baseline).
Moreover, the polynomial method was used to determine the
contact angle. In the polynomial method, the parameters of a
polynomial function are fitted to the drop contour. The drop is
unsymmetrical so contact angles are measured on both sides of the
liquid drop profile, and the values are averaged.
I=I0 ¼ expð ꢁ d=l sin qÞ;
(1)
where d is the layer thickness,
substrate-specific photoelectron in the organic layer,
l
the mean free path of the
q the analysis
take-off angle relative to the surface, and I/I0 the ratio of the Au4f7/2
peak intensities for the modified (I) and bare Au surface (I0). Details
of this calculation are reported in the Supporting Information. Fig. 4
plots the variations of the thickness of the iniferter layer as a
function of time, estimated from XPS data, as well as the (C þ O)/Au
atomic ratios, representative of the chemical composition of the
organic coating.
From these results, it appears that, contrary to alkylthiols mol-
ecules which tend to form monolayers on surfaces [46], the
diazonium-derived iniferter layers grow progressively with time,
forming functional poly(phenylene) multilayers as previously re-
ported in the literature [47]. The thickness of these iniferter layers
varies from less than 1 nm after 15 min of incubation to almost
20 nm after 24 h of incubation.
3. Results and discussion
3.1. Grafting of aryl-terminated iniferters on planar gold surface
A comprehensive study of the chemical nature of the interface
was performed by closely examining the high resolution spectra of
all the elements present in the XPS survey spectrum at low iniferter
layer thickness (after incubation time of 15 min). Three types of
chemical bonding between the Au substrate and the iniferter
grafted groups were considered: (i) the interaction of Au with the
sulfur atoms from DEDTC units; (ii) the direct reaction of diazonium
salts with the gold surface leading to AueN interfacial bonds, as
already observed by G. Deniau et al. [48], or (iii) the presence of
AueC bonds explained by the grafting of aryl cations or radicals on
the surface, arising from dediazoniation.
The high resolution spectra of sulfur S2p and nitrogen N1s are
displayed in Fig. 5. The S2p signal, at 163.8 eV is characteristic of
DEDTC units which do not interact with Au. This highlights the
absence of AueS interaction (no visible peak at ca. 162 eV). The N1s
signal, at 400.3 eV is also characteristic of DEDTC units. The absence
of any signal at lower binding energy (around 397.5 eV) indicates
that the grafting of the iniferter molecules is not based on AueN
The spontaneous grafting of the diazonium-derived iniferter
layer at the surface of gold was performed by simply incubating the
substrates with the corresponding diazonium salts in water at room
temperature. Strong modifications in surface chemical composition
were observed by XPS after functionalization of the gold substrates.
Indeed, the gold signal appears strongly attenuated while the car-
bon and oxygen content increase and new peaks assigned to sulfur
and nitrogen appear, indicating the covering of gold by the iniferter
layers (see Table 1). Depending on the size of the oligo(ethylene
oxide) spacers between the diazonium anchoring moieties and the
iniferter groups, the proportion of oxygen changes, increasing for
longer spacers.
The peak fitting of the C1s signal displayed in Fig. 3 clearly
reveals all the components contained in the grafted iniferter
layers. The intense peak at 285 eV is assigned to CeC and C]C
environments, the one at 286.6 eV corresponds to the CeO
Ahmad, Randa
