Application of visible light photocatalysis with particle lithography to generate polynitrophenylene nanostructures

Article abstract of DOI:10.1021/ja505521k

Visible light photoredox catalysis was combined with immersion particle lithography to prepare polynitrophenylene organic films on Au(111) surfaces, forming a periodic arrangement of nanopores. Surfaces masked with mesospheres were immersed in solutions of p-nitrobenzenediazonium tetrafluoroborate and irradiated with blue LEDs in the presence of the photoredox catalyst Ru(bpy)₃(PF₆)₂ to produce p-nitrophenyl radicals that graft onto gold substrates. Surface masks of silica mesospheres were used to protect small, discrete regions of the Au(111) surface from grafting. Nanopores were formed where the silica mesospheres touched the surface; the mask effectively protected nanoscopic local areas from the photocatalysis grafting reaction. Further reaction of the grafted arenes with aryl radicals resulted in polymerization to form polynitrophenylene structures with thicknesses that were dependent on both the initial concentration of diazonium salt and the duration of irradiation. Photoredox catalysis with visible light provides mild, user-friendly conditions for the reproducible generation of multilayers with thicknesses ranging from 2 to 100 nm. Images acquired with atomic force microscopy (AFM) disclose the film morphology and periodicity of the polymer nanostructures. The exposed sites of the nanopores provide a baseline to enable local measurements of film thickness with AFM. The resulting films of polynitrophenylene punctuated with nanopores provide a robust foundation for further chemical steps. Spatially selective binding of mercaptoundecanoic acid to exposed sites of Au(111) was demonstrated, producing a periodic arrangement of thiol-based nanopatterns within a matrix of polynitrophenylene.

Full text of DOI:10.1021/ja505521k

Article
pubs.acs.org/JACS
Application of Visible Light Photocatalysis with Particle Lithography
To Generate Polynitrophenylene Nanostructures
Susan D. Verberne-Sutton, Rashanique D. Quarels, Xianglin Zhai, Jayne C. Garno, and Justin R. Ragains*
Department of Chemistry, Louisiana State University, 232 Choppin Hall, Baton Rouge, Louisiana 70803, United States
S
* Supporting Information
ABSTRACT: Visible light photoredox catalysis was combined with immersion particle lithography to prepare polynitro-
phenylene organic films on Au(111) surfaces, forming a periodic arrangement of nanopores. Surfaces masked with mesospheres
were immersed in solutions of p-nitrobenzenediazonium tetrafluoroborate and irradiated with blue LEDs in the presence of the
photoredox catalyst Ru(bpy)3(PF6)2 to produce p-nitrophenyl radicals that graft onto gold substrates. Surface masks of silica
mesospheres were used to protect small, discrete regions of the Au(111) surface from grafting. Nanopores were formed where
the silica mesospheres touched the surface; the mask effectively protected nanoscopic local areas from the photocatalysis grafting
reaction. Further reaction of the grafted arenes with aryl radicals resulted in polymerization to form polynitrophenylene
structures with thicknesses that were dependent on both the initial concentration of diazonium salt and the duration of
irradiation. Photoredox catalysis with visible light provides mild, user-friendly conditions for the reproducible generation of
multilayers with thicknesses ranging from 2 to 100 nm. Images acquired with atomic force microscopy (AFM) disclose the film
morphology and periodicity of the polymer nanostructures. The exposed sites of the nanopores provide a baseline to enable local
measurements of film thickness with AFM. The resulting films of polynitrophenylene punctuated with nanopores provide a
robust foundation for further chemical steps. Spatially selective binding of mercaptoundecanoic acid to exposed sites of Au(111)
was demonstrated, producing a periodic arrangement of thiol-based nanopatterns within a matrix of polynitrophenylene.
phenyl,21 and 4-decylbenzene.27 With further chemical steps
INTRODUCTION
■
the grafted films can be used to build multilayered surface
Covalently bound films produced with electrochemical
reduction of diazonium salts are more stable than surface
layers produced by chemisorption of n-alkanethiols, providing
structures.30−33
Photochemical approaches with diazonium salts have not
been widely investigated for grafting organic films to surfaces.34
benefits as electrochemical sensor surfaces that require long-
term storage or repeated chemical cycling.1,2 A limitation of
Visible light photocatalysis can be used with diazonium salts in
the presence of a photoredox catalyst to produce carbon-
electrografting is that the surface must be comprised of
centered radicals that undergo processes that include grafting to
conductive or semiconductor materials. Electrochemical
both conducting and non-conducting surfaces under mild, user-
reduction of salts has been used to graft organic films to
friendly conditions.34−38 Visible light photocatalytic grafting of
arenediazonium salts was accomplished by Bouriga et al. using
the photocatalyst Ru(bpy)3(PF6)2 with both Au(111) and the
surfaces of gold,3−6 iron,7 silicon,8,9 carbon,10−18 diamond,19
platinum,20 indium−tin−oxide,21,22 and highly oriented py-
rolytic graphite23,24 electrodes in either organic or aqueous
media. Electrochemical reduction of arenediazonium salts was
accomplished with Co, Ni, Cu, Zn, Pt, and Au substrates.25 The
surface layers generated by electrochemical reduction are
typically less than 10 nm in thickness and attach covalently
to the electrode. Organic films formed from reduction of
diazonium salts offer flexibility for selecting diverse molecules
to design surface properties and reactivity. A broad range of
derivatives of diazonium salts have been electrografted,
including those of tetraarylporphyrin,11 anthraquinone,4,5
bithiophene phenyl,26 4-carboxybenzene,24 4-nitro-
benzene,3,9,10,16,18−21,27−29 4-aminobenzene,6,14 4-fluorene-
non-conductive polymer surface of polyvinyl chloride.34
A
similar visible light photocatalytic approach for the surface-
patterning of coumarin-modified cellulose was reported by
Schroll et al.35 Irradiation with UV light was used to
photochemically reduce 4-(2-(4-pyridinyl)ethynyl)benzene-
diazonium salt to form a film on quartz substrates, as reported
by Zhao et al.31 Ultraviolet irradiation of the charge transfer
complex of 1,4-dimethoxybenzene and the nitrobenzene-
Received: June 2, 2014
Published: September 22, 2014
© 2014 American Chemical Society
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aqueous solution of 4-nitroaniline and HBF4 over a period of 30 min
under N2 at 0 °C. As the resulting cloudy green mixture became
difficult to stir, the stirring rate was increased. The round-bottom flask
was wrapped in foil, and the mixture was stirred for an additional 1 h at
0 °C. The resulting mixture was filtered and washed with 2 mL of cold
deionized water. The resulting solid was added to a 250 mL
Erlenmeyer flask and dissolved in 30 mL of acetone. The remaining
precipitate was removed by filtration and washed with 2 mL of
acetone. Twenty milliliters of anhydrous diethyl ether was added at
once to the resulting filtrate. The precipitated diazonium salt was
diazonium cation resulted in the grafting of metal surfaces (Au,
Cu, Fe), reported by Busson et al.39
Surface patterning has been accomplished with organic films
produced by electrochemical reduction of diazonium salts using
methods developed with scanning electrochemical microscopy
(SECM),40 microcontact printing,41,42 scanning probe-based
nanolithography,9,10,43 electrografting,44 and particle lithogra-
phy.21,33,45,46 The combination of particle lithography with
molecular surface assembly is a practical strategy for preparing
periodic nanostructures with high throughput.47
filtered using a Buchner funnel, washed with 10 mL of anhydrous
̈
diethyl ether, and dried under high vacuum for 1 h. Due to its low
stability at room temperature, the diazonium salt was used
immediately or stored at −2 °C prior to use. A total of 1.2344 g
(52.1%) of C6H4N3O2BF4 was isolated. 1H NMR (400 MHz,
CD3CN): δ 8.74 (d, 2H, J = 9.32 Hz), 8.63 (d, 2H, J = 9.32 Hz).
13C NMR (100 MHz, CD3CN): δ 155.3, 135.5, 126.7, 121.6. The
In the work described herein, visible light photocatalysis was
combined with particle lithography to successfully generate
films of polynitrophenylene with designed thickness on
Au(111). The films of polynitrophenylene were prepared by
visible light irradiation of Ru(bpy)3(PF6)2 in the presence of 4-
nitrobenzenediazonium tetrafluoroborate. Our strategy for
nanofabrication applied particle lithography with surface
masks consisting of silica mesospheres (d = 500 nm) to
protect small, discrete regions of the surface from the
photocatalytic grafting reaction. After steps of photografting,
the exposed sites of nanopores were backfilled with 11-
mercaptoundecanoic acid (MUA) to generate a composition-
ally patterned surface composed of MUA nanostructures within
a matrix film of polynitrophenylene.
NMR spectra can be found in Supporting Information, Figures S1 and
S2.
Photografting Procedure. Acetonitrile (MeCN) from a solvent
purification system48 was dried over 3 Å molecular sieves for 24 h. A
solution of NBDT (47.4 mg, 0.2 mmol) and Ru(bpy)3(PF6)2 (3.0 mg,
0.005 mmol) in 2 mL of anhydrous MeCN was wrapped in foil,
stirred, and deoxygenated three times using the freeze−pump−thaw
protocol. A lower concentration (10−3 M) of NBDT was prepared via
dilution. A 20 μL aliquot of the 10−1 M solution of NBDT was
transferred to a separate round-bottom flask wrapped in foil.
Anhydrous MeCN (1.98 mL) was then added to achieve the desired
concentration (10−3 M). This solution was also deoxygenated three
times using the freeze−pump−thaw protocol. An egg-shaped stir bar
and the masked gold substrate were physically separated inside of a
125 mL Erlenmeyer flask so that the stir bar did not touch the masked
gold substrate. The Erlenmeyer flask was sealed with a rubber septum
and purged with a continuous flow of nitrogen for 30 min (see
Supporting Information, Figure S3). The deoxygenated solution of
NBDT and Ru(bpy)3(PF6)2 in anhydrous MeCN was added to the
125 mL Erlenmeyer flask containing the masked gold sample and stir
bar. The suspension was magnetically stirred and irradiated. After
irradiation for the desired period of time, the solution was decanted.
The masked gold sample was washed twice with 2 mL of deionized
water and twice with 2 mL of ethanol to remove material that was not
strongly attached to the gold surface. The samples were sonicated in
ethanol for 1 min to remove the mesospheres and any remaining non-
covalently bound residues.
EXPERIMENTAL SECTION
■
Materials and Reagents. Sodium nitrite (99.8% ACS certified)
was obtained from Fisher Scientific. Ru(bpy)3(PF6)2 (97%), 4-
nitroaniline (99%), tetrafluoroboric acid (48% aqueous), anhydrous
diethyl ether (99.0% ACS reagent grade), acetone (99.5% ACS reagent
grade), and mercaptoundecanoic acid (95%) were purchased from
Sigma-Aldrich and used without further purification. 1H NMR and 13
C
NMR spectroscopy were performed on a Bruker AV-400 spectrom-
eter. Unless otherwise noted, all materials were purchased from Sigma-
Aldrich and used without further purification. Glassware was flame-
dried under vacuum and backfilled with dry nitrogen prior to use.
Deuterated solvents were obtained from Cambridge Isotope Labs.
Acetonitrile for grafting procedures was purified according to the
method published by Pangborn et al.48 The irradiation source for
photografting reactions was two 4W sapphire blue LED flex strips
from Creative Lighting Solutions (Cleveland, OH).
Backfilling the Nanopores with Mercaptoundecanoic Acid
(MUA). A solution of MUA in ethanol was prepared by adding 0.0014
g of MUA into 10 mL ethanol and stirring until it was completely
dissolved (0.6 mM). The solution was injected into a liquid cell
(Sonimoto Laboratories, West Bloomfield, MI) that supports imaging
samples in a liquid environment. Fresh aliquots (1 mL) of the MUA
solution were replenished each hour to replace liquid lost by
evaporation.
Atomic Force Microscopy (AFM). The samples were imaged
using a model 5500 or model 5420 atomic force microscope with Pico
View v.1.12 software (Agilent Technologies, Chandler, AZ). Non-
conductive imaging probes from Bruker (MSCT, 0.01−0.6 N/m) were
used to acquire contact-mode images. Images were processed and
analyzed using Gwyddion (v. 2.22), an open-access processing
software designed for AFM images, supported by the Czech Metrology
Institute.51
Preparation of Gold Surfaces with a Mesosphere Mask. Two
types of substrates were used for preparing samples. Ultraflat gold
films were prepared on mica(0001) by evaporative deposition.
Template-stripped, gold substrates were prepared by a previously
reported procedure.49 Glass discs were glued to freshly prepared gold
films using an epoxy (EPO-TEK, Billerica, MA). Pieces of ultraflat
gold/glass were stripped from mica to expose a clean, atomically flat
Au(111) surface. Size-sorted silica mesospheres with an average
diameter of 500 nm (Thermo Scientific) were cleaned by
centrifugation and suspension in water (three cleaning cycles). A 40
μL drop of the silica mesosphere suspension was placed onto the
template-stripped gold substrates, dried in air for 2 h, and then oven-
dried at 150 °C for at least 14 h. The final heating step was used to
temporarily anneal the silica spheres to the substrate to prevent
displacement during chemistry steps of immersion in solutions. After
completion of the steps of the chemical reactions, removal of the
surface mask was accomplished by sonication in clean solvents.
Preparation of 4-Nitrobenzenediazonium Tetrafluoroborate
(NBDT). The NBDT was prepared according to a method similar to
that reported by Vogel using 4-nitroaniline.50 Tetrafluoroboric acid
(HBF4, 3.6 mL, 27 mmol) was added to a round-bottom flask
containing 5 mL of deionized water. A solution of 4-nitroaniline (1.40
g, 10 mmol) was added in three portions to the round-bottom flask
and stirred. The resulting olive-green solution was cooled to 0 °C in an
ice−water bath. A solution of sodium nitrite (0.69 g, 10 mmol) in
deionized water (2 mL) was prepared and then added dropwise to the
RESULTS AND DISCUSSION
■
To accomplish photografting, gold surfaces masked with
mesospheres were immersed in solutions of NBDT in MeCN
and irradiated for selected time intervals in the presence of the
photocatalyst Ru(bpy)3(PF6)2 to produce p-nitrophenyl
radicals that grafted onto Au(111). As the reaction proceeds,
successive attachment of aryl radicals to grafted arenes
generated films of polynitrophenylene (Scheme 1). The mask
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Scheme 1. Photografting Reaction Steps
of mesospheres was removed by ultrasonication in ethanol to
generate a periodic arrangement of uncovered nanopores in
locations where the silica mesospheres covered the substrate.
For scanning probe studies, the nanopores provide a useful
baseline for measuring the thickness of the films and also
provide exposed surfaces sites of the substrate for backfilling
with selected molecules.
Photografting of NBDT onto Masked Gold Substrates.
A densely packed film of polynitrophenylene was formed after
30 min of irradiation at a concentration of 10−3 M NBDT,
shown in Figure 1. A periodic arrangement of dark holes is
not exhibit molecular vacancy islands that are characteristic of
Au surface reconstruction typically observed with n-alkanethiol
monolayers.52 The dense film has 77 nanopores within the
topograph (Figure 1a) spaced at 500 nm distances,
corresponding to the diameter of the surface mask. A hexagonal
arrangement of seven nanopores is shown in Figure 1b. The
shapes are not perfectly round and symmetric at the nanoscale;
rather the roughness and slight imperfections of the underlying
substrate influence the shape. The nanostructures with regular
circular geometries occur at sites of flat terraces without scars or
terrace edges. A magnified view of an individual nanopore is
shown in Figure 1c, and an example cursor profile across the
nanopore is shown in Figure 1d. The images in Figure 1 are
representative of multiple areas of the sample. The areas of
nanopores provide an opportunity to obtain multiple measure-
ments of the local film thickness to obtain a representative
value. The average depth measured 2.6 0.2 nm (n = 40) for
the polyphenylene film prepared with the selected reaction
conditions. The average value was derived by obtaining a
representative cursor measurement across the central area of
individual nanopores acquired along the horizontal scan
direction of the image, n refers to the number of nanopores
measured.
Effect of Increased NBDT Concentration for Film
Thickness. One would predict that increasing either the
concentration or the irradiation time would correspondingly
produce a thicker film of polynitrophenylene. To test this
hypothesis, further experiments with particle lithography were
conducted to evaluate the parameters of concentration and
light exposure while keeping the other parameters unchanged.
The effects of increasing the concentration by 100-fold (0.1 M
NBDT) after a 5 min irradiation time are shown in Figure 2.
The morphology of the film appears relatively smooth and
dense. The domain boundaries and scars of the underlying
substrate are not evident for the sample in Figure 2, which was
prepared using template-stripped gold. The process of gluing
glass to the ultraflat gold film and stripping it from the mica
substrate provided a distinctly flat and uniform surface
morphology. For thicker surface films, the substrate features
are likely to be indistinguishable. There are 91 nanopores
within the 6 × 6 μm2 topography image shown in Figure 2a.
The uncovered sites became more prominent, with clearly
resolved edges. A comparison of surface chemistry is mapped in
lateral force images rather than changes in height. Correspond-
ing differences in frictional forces between the AFM probe and
surface groups are mapped in the lateral force image (Figure
2b). The bright areas are the uncovered areas of the gold
substrate measuring ∼18% coverage of the surface. The areas of
the polynitrophenylene film are darker and homogeneous in
color, suggesting a fairly uniform chemical composition
throughout the sample. A close-up view of three nanopores is
Figure 1. Photografted film of polynitrophenylene formed on Au(111)
generated with an initial concentration of 1 mM NBDT, after 30 min
of irradiation. Nanopores were produced using immersion particle
lithography. (a) Topography image (5 × 5 μm2) acquired in air; (b)
zoom-in view (1.5 × 1.5 μm2); (c) a single nanopore (500 × 500
nm2); (d) cursor profile for the line in c.
observed throughout areas of the sample. The areas of the
nanopores are sites of uncovered substrate where the
mesoparticle mask was removed. Approximately 1% of the
surface is covered with nanopores in Figure 1A, scaling to a
surface density of 108 nanopores/cm2. The sample was
prepared with an NBDT concentration of 1 mM and was
imaged with contact-mode AFM in ambient air. Features such
as terraces, scars, and domain boundaries that are characteristic
of the underlying Au(111) substrate are evident within the 5 ×
5 μm2 topography image of Figure 1a. For well-resolved AFM
images of thin organic films the features of the underlying
Au(111) substrate such as terraces, scars, and domain
boundaries can often be detected. For thicker films such
features will no longer be resolvable. Interestingly, the films do
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Figure 3. Photografted film of polynitrophenylene punctuated with
nanopores prepared with 15 min irradiation in 0.1 M NBDT. (a)
Topography image (5 × 5 μm2); (b) zoom-in topograph (750 × 750
nm2); (c) cursor profile across two nanopores in panel b.
Figure 2. Nanopores within a polynitrophenylene film formed with 0.1
M NBDT after 5 min of irradiation. (a) Topography and (b)
corresponding lateral force images (6 × 6 μm2); (c) zoom-in view of
nanopores (750 × 750 nm2); (d) cursor profile for the line in panel c.
shown in Figure 2c, revealing a regular and smooth morphology
for the photografted film. The diameter of the nanopores
measured 77
experiment in Figure 1. The depths of the nanopores measured
0.2 nm (n = 49), shown with an example cursor profile in
14 nm (n = 38), which closely matches the
3
Figure 2d.
In previous reports with electrochemically grafted films, the
compactness or density was greater with increasing diazonium
salt concentration.43,53 With electrochemical grafting, high
concentrations of diazonium salts can produce multilayer films
after longer reduction times.54 Comparing the two experiments
shown in Figures 1 and 2, changing the parameters of
immersion time, as well as increasing the diazonium and
catalyst concentrations, did not greatly increase the overall film
thickness; however, the film in Figure 2 appears to be more
dense and compact.
Effect of Increased Irradiation Time for Film Thick-
ness. The parameter of increasing the irradiation time was
tested using 0.1 M NBDT for the experiment shown in Figure
3. After 15 min of irradiation, AFM images of the polynitro-
phenylene film revealed a less regular topography and thicker
layer (Figure 3a). Thicker areas formed an arrangement of
strips across areas of the sample with areas of flatter domains in
between the ridges. Zooming-in for a closer view, the areas of
polymer have a rougher, clustered morphology (Figure 3b).
The nanopores can still be distinguished as dark spots
throughout areas of the sample within a dense film. A
representative line profile across two of the nanopores is
shown in Figure 3c, with the thickness of the film measured to
be 13 2 nm (n = 49).
When the reaction time was further increased to 30 min
while maintaining an initial concentration of 0.1 M NBDT, the
film morphology and thickness changed substantially (Figure
4). The morphology of the thicker film of polynitrophenylene is
no longer smooth and continuous. A rougher, irregular
arrangement of surface structures surrounding nanopores is
apparent in Figure 4a. Within a magnified view (Figure 4b), a
honeycomb arrangement of seven nanopores is disclosed with
polymer clusters surrounding the bowl-shaped nanopores. The
Figure 4. Polynitrophenylene film prepared with an irradiation time of
30 min in 0.1 M NBDT. (a) Topography image (6.5 × 6.5 μm2)
acquired in air; (b) zoom-in view (1.25 × 1.25 μm2); (c) cursor
profiles for the lines in panel b.
thickness of the film measured 89
22 nm (n = 46). An
example cursor profile is shown in Figure 4c; however, the
irregularly shaped edges of nanopores cannot be fully
represented with a single line scan. Therefore, an additional
line trace is presented for an area without nanopores. Using the
bottom of the nanopores as a baseline, the range of heights
measured from 57 to 163 nm (n = 46).
By changing the duration of irradiation for time intervals on
the order of a few minutes, considerable increases in film
thickness were detected. A comparison of the film thickness for
selected irradiation times is shown in Figure 5, referencing the
bottom of nanopores as a baseline. The error bars were
determined from the standard deviation of the measurements.
The chart effectively demonstrates the sensitivity to changes in
the duration of light exposure for the photografting reaction
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Figure 5. Thickness of the polynitrophenylene films measured at
different irradiation times for samples prepared with 0.1 M NBDT.
with diazonium salts. As irradiation time was increased, the
thickness of the film increased within an overall time frame of
30 min. For the parameter of the concentration of NBDT, the
differences in the thickness of the film were not as large. The
film formed with a starting concentration of 0.001 M NBDT
measured 2.6
0.2 nm in thickness (30 min irradiation),
whereas the film prepared with 0.1 M NBDT (5 min
irradiation) measured 3.2 0.2 nm. The difference in thickness
of 0.6 nm corresponds approximately to the addition of a single
phenylene group.
Figure 6. After backfilling with an n-alkanethiol, nanostructures
formed within exposed sites of the nanopores. (a) Topography image
of nanopores within a matrix film of polynitrophenylene (2 × 2 μm2);
(b) simultaneously acquired lateral force image; (c) topography after
backfilling with MUA (2 × 2 μm2); (d) corresponding lateral force
image.
Two control experiments were run to evaluate the progress
of the reaction without catalyst (Supporting Information,
Figure S4) and without illumination (Supporting Information,
Figure S5). The spontaneous grafting of the diazonium salt was
evaluated using 0.1 M NBDT for 30 min reaction time. The
control sample prepared without catalyst (Figure S4) did
evidence spontaneous grafting of polyphenylene at areas near
the meniscus sites of the mesosphere masks, with thicknesses
ranging from 1 to 3 nm. The control sample prepared without
visible light illumination (Figure S5) showed evidence for the
spontaneous grafting of polyphenylene at areas throughout the
sample, measuring 7 2 nm in thickness.
Grafting of diazonium salts using electrochemical reduction
is primarily a diffusion-controlled process.4 Reduction of the
diazonium cation by the substrate produces an aryl radical that
couples to the surface for growth of an organic film. Multilayers
are generated as the reaction proceeds with phenyl radicals
attaching to the surface-bound molecules, forming covalent
carbon−carbon bonds.54 Once the multilayer film has reached a
thickness at which electron transfer through the surface film is
no longer possible, the growth will terminate.18 With
electrochemical reduction, additional surface layers can restrict
reduction of further molecules at the surface. Photocatalysis, on
the other hand, continuously produces radicals to graft either to
the surface or to grafted molecular layers to effect continuous
growth under conditions of irradiation with visible light. The
growth can effectively be switched off by removing the light
source. In our experiments, an increase in film thickness was
observed when the duration of light exposure was increased,
while keeping all other parameters constant. Surface layers up
to 100 nm in thickness are accessible with catalytic photo-
grafting because the growth of multilayers is not limited by a
reduction in conductivity as it is with electrografting.55
arrangement of 16 nanopores within the film of polynitro-
phenylene. The shapes of the nanoscopic surface sites of
exposed Au(111) can be clearly distinguished in the lateral
force frame (Figure 6b). After 24 h of immersion in MUA,
clusters of molecules attached within the exposed areas of the
sample (Figure 6c). A few of the sites have multiple islands
within the nanopores which can be resolved in the lateral force
image (Figure 6d). After backfilling, the area that was selected
for imaging has shifted; the frames in Figure 6c,d contain 14
nanostructures within an area of 2 × 2 μm2, which would scale
to a surface density of ∼108 nanopores/cm2.
A side-by-side comparison of the stability of the diazonium-
derived film versus thiol-bound MUA was accomplished with
the backfilled sample of Figure 6. An experiment was designed
to use the force of an AFM probe to shave regions of the
sample. Thiolated molecules can be displaced from the surface
using a process known as nanoshaving.56 Local areas of surface
films can be shaved away to expose the substrate by increasing
the force on the probe. Steps of the nanoshaving process are
shown with AFM topography frames in Figure 7. An example
image of a single nanostructure of MUA is shown in Figure 7a.
Before backfilling with MUA, the depth of the nanopore
measured 3.0 0.2 nm (n = 49). Under low force, images can
be acquired without disturbing the shapes of the nanostruc-
tures. After immersion in MUA solution for 24 h, a
nanostructure measuring 8.7 nm was formed (Figure 7b).
The height corresponds to the thickness of a multilayer of
MUA formed by interactions between acid headgroups.
Nanostructures of multiple layers of MUA were previously
observed to form under certain conditions as reported by
Kelley et al.57 The area containing the nanostructure and
surrounding matrix film (500 × 500 nm2) was scanned at
higher force (swept 20 times) to shave away the surface film.
The force was sufficient to remove thiol molecules; however,
the photografted film of polynitrophenylene persisted, as shown
in the representative image in Figure 7c. Some of the displaced
molecules from the nanostructure remained at the edge of the
Backfilling Exposed Sites of Au(111) with Mercapto-
undecanoic Acid. The exposed surfaces sites of the substrate
located within the nanopores can be filled with selected
molecules to generate nanostructures with designed chemical
groups. An ω-functionalized n-alkanethiol which contains a
carboxylic acid headgroup was chosen for backfilling. MUA in
ethanol (0.6 mM) was added to the sample shown in Figure 2
(0.1 M NBDT, 5 min irradiation). A close-up view of the
sample is presented in Figure 6a, revealing the shapes and
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Figure 7. Steps for backfilling and shaving the nanopores filled with MUA. Topography images (500 × 500 nm2) of (a) a single nanopore within the
polynitrophenylene matrix film; (b) nanostructure of MUA formed within a nanopore; (c) the same area of the MUA nanostructure after
nanoshaving. The local areas of nanopore are not the same for the ex situ experiments.
pattern. The thickness of the polynitrophenylene film after
nanoshaving (Figure 7c) matches the thickness measured
before nanoshaving (Figure 7a).
without illumination, respectively. This material is available free
of charge via the Internet at http://pubs.acs.org.
Under certain experimental conditions, aryl films formed
from the reduction of diazonium salts were bonded to gold
surfaces more strongly compared to equivalent thiol-bound
films, as reported by Shewchuk et al.2 For the experiment in
Figure 7, after nanoshaving, the polynitrophenylene film
persisted on the gold surface while the MUA nanostructure
was removed by the sweeping action of the AFM tip. The
photografted film provided spatial selectivity as a matrix film.
No evidence of self-exchange or adsorption of MUA on areas
between the nanostructures was visible throughout the course
of a 24 h experiment.
AUTHOR INFORMATION
■
Corresponding Author
jragains@lsu.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge support from the National
Science Foundation Career/PECASE award (CHE-0847291),
the American Chemical Society Petroleum Research Fund New
Directions Program (52305-ND), the Camille Dreyfus
Teacher-Scholar Program, and the Louisiana Board of Regents
RCS support fund (LEQSF(2013-16)-RD-A-03). We thank
Ms. Chelsea Eaton for assistance with the synthesis of NBDT.
CONCLUSIONS
■
Nanopores within polynitrophenylene films on Au(111)
surfaces were prepared using visible light photocatalysis
combined with particle lithography. To accomplish nano-
lithography, substrates were masked with a film of silica
mesospheres to protect local areas of the surface from polymer
deposition. Solutions of p-nitrobenzenediazonium tetrafluoro-
borate and the photoredox catalyst Ru(bpy)3(PF6)2 were
irradiated to produce p-nitrophenyl radicals that graft onto
masked substrates of Au(111). Within only a few minutes of
irradiation, samples were generated with exquisite arrangements
of periodic nanopores within densely packed films of
polynitrophenylene. The depth of nanopores was used to
evaluate film thickness, which was found to depend sensitively
on the parameter of irradiation time. The nanopores were filled
with a second molecule with thiol−gold chemisorption (MUA),
generating a surface with designed interfacial chemistry.
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