The variation in pore size in mesoporous films produced by electrochemically assisted self-assembly (EASA) with the surfactant chain length is described. EASA produces a hexagonal array of pores perpendicular to the substrate surface by using an applied potential to organize cationic surfactants and the resultant current to drive condensation in a silica sol. Here, we show that a range of pore sizes between 2 and 5 nm in diameter is available with surfactants of the form [Me3NCnH2n+1]Br, with alkyl chain lengths between C14 and C24. The film quality, pore order, pore size, and pore accessibility are probed with a range of techniques.
The variation in pore size in mesoporous films produced by electrochemically assisted self-assembly (EASA) with the surfactant chain length is described. EASA produces a hexagonal array of pores perpendicular to the substrate surface by using an applied potential to organize cationic surfactants and the resultant current to drive condensation in a silica sol. Here, we show that a range of pore sizes between 2 and 5 nm in diameter is available with surfactants of the form [Me3NCnH2n+1]Br, with alkyl chain lengths between C14 and C24. The film quality, pore order, pore size, and pore accessibility are probed with a range of techniques.
Oriented mesoporous
silica films are potential hard templates for
the electrodeposition of nanowire-based devices,[1] with possible applications in Li-ion batteries,[2,3] thermoelectrics,[4] and supercapacitors.[5] In order for the electrode surface to remain
accessible to the electrolyte it is necessary to arrange the pores
perpendicular to the substrate surface, often referred to as the vertical
orientation. Although the growth of platinum nanowires in silica with
horizontal pores occurred readily, Kanno et al.[6] showed that the electrodeposition of Au nanowires was much
easier in perpendicular pores.[7] While the
literature on electrodeposition in very small pore templates is limited,
there are some examples, for example, of 3 nm Sn nanowires[8] or 5 to 8 nm bismuth nanowires,[9] produced in this way.Pore orientation, pore diameter,
and organo-functionalization[10,11] are important factors
in the rate of ion diffusion through porous
films. Etienne and co-workers reported TiO2 with the anatase
structure in an open porous network, in which positively charged species
were more mobile than negatively charged ones.[12] The fine tuning of parameters has made silica films attractive
materials for biosensing and electrochemical sensing applications.[13,14] For example, Nasir et al.[15] developed
an electrochemical pathway for detecting cationic paraquat salts in
oriented mesoporous silica films.The most common approach to
preparing mesoporous silica films is
using evaporation-induced self-assembly (EISA).[16−18] This method
involves the preparation of an aqueous ethanol solution containing
a surfactant and silica precursors to produce a homogenous solution
of stable species. The initial surfactant concentration of the prepared
solution is significantly lower in comparison to the critical micelle
concentration (Co ≪ CMC). Once
the films have been deposited in solution by either dip or spin coating,
the ethanol solvent evaporates from the deposited film, resulting
in the surfactant concentration increasing, which in turn triggers
the self-assembly process and micelle formation, producing well-organized
porous structures in the liquid crystalline phase before condensing
the silica species. EISA tends to produce ordered silica films with
the pores either horizontal to the substrate surface or randomly oriented.Several strategies have been devised to produce orientated silica
films through the application of a magnetic field,[19] epitaxial growth,[20] and radio
sputtering.[21] For example, Yamauchi used
an external magnetic field to control pore alignment from an EISA
process either vertically or horizontally to the substrate surface.[22] Otomo et al.[23] synthesized
aligned mesoporous silica films by a radiofrequency sputter deposition
of Co–Si–O films where the surface morphology was influenced
by a range of argon gas pressures followed by wet chemical etching
to remove the Co fragment from the films. Stöber-solution growth
is based on the formation of hemispherical micelles of a cationic
surfactant on a negatively charged substrate and interactions with
silica oligomers that encourage the vertical growth of those micelles.[24] This method can take days to grow very thin,
aligned films.Walcarius and co-workers[25] revealed
that the preparation of vertically oriented mesoporous silica films
was possible under electrochemical control. The mechanism for the
electrochemically assisted self-assembly (EASA) of silica comprises
of applying a negative potential to the substrate to be coated while
it sits in a solution containing the surfactant and silica precursors
dissolved in an ethanol/water mixture. This results in the generation
of hydroxide ions in the solution, and the pH swings from acidic to
alkaline at the electrode surface. The electric field is thought to
play a pivotal role in the self-organization of the cationic surfactant
into hemimicelles, and the pH change catalyzes the polycondensation
of the sol, leading to mesoporous silica films with perpendicular
(vertical) pore channels (2–3 nm) and uniform film thickness
(150–200 nm). Mesoporous films can be obtained using the EASA
method within as little as 20 s.EASA was developed with a [Me3NC16H33]Br (CTAB or C16TAB)
surfactant, and this leads to pores
with a diameter of around 2 nm depending on conditions and substrates.[26] Robertson et al. showed that mesitylene can
be used as a swelling agent for CTAB-templated EASA films on TiN,
causing the pore diameter to increase from 1.6 to 2.4 nm.[27] However, larger still amounts of mesitylene
resulted in a reduction in the order of the porosity. Larger vertically
aligned pores may facilitate the use of such films as hard templates
for nanowire electrodeposition and increase the range of sensing applications.
Ullah et al. recently showed that polyaniline nanowires with different
widths could be produced in EASA silica films produced with C16TAB or C18TAB.[28] Herein,
we demonstrate that systematic increases in the pore diameter beyond
2 nm are achievable by EASA when utilizing surfactants created by
further extending the single straight alkyl chain.
Experimental Section
Tetradecyltrimethylammonium bromide
(C14TAB, 99%), hexadecyltrimethylammonium
bromide (C16TAB, 98%), and octadecyltrimethylammonium bromide
(C18TAB, 98%) were purchased from Sigma-Aldrich. The 1H and 13C{1H} nuclear magnetic resonance
(NMR) data for the surfactants in d-chloroform were
collected using a Bruker AVII400 spectrometer at 25 °C. Solutions
for mass spectrometry were prepared by dissolving 50 μg of the
surfactant in 1 mL of methanol. The surfactants were analyzed using
ultrahigh performance liquid chromatography coupled to a TQD mass
tandem quadrupole mass spectrometer (Waters) using a TUV detector
at 254 nm. The chromatography analysis was carried out using a Waters
BEH C18 column (50 mm × 2.1 mm, 1.7 μm) with a flow rate
of 0.6 mL/min. The mobile phase was made up of 0.2% formic acid in
an aqueous solution and 0.2% formic acid in acetonitrile. This was
followed by the recording of the mass spectrum in the positive ion
electrospray ionization mode.The synthesis of longer chain
alkyltrimethylammonium bromide surfactants
(Scheme ) was carried
out according to Scheraga et al.[29] The
synthesis used alkyl bromides CH2Br where n is equal to
20 (5.0001 g, 13.83 mmol ≥ 97%, Sigma-Aldrich), 22 (5.0001
g, 12.84 mmol, 96%, Santa Cruz Biotechnology), or 24 (0.8915 g, 2.14
mmol, synthesized as described below). The alkyl bromide was dissolved
in ethanol (25 cm3) and 30% trimethylamine in an ethanol
solution (7.6000 g, 128.6 mmol for C20 and C22, or 1.3000 g, 22.0 mmol for C24, Sigma-Aldrich) was added
dropwise into this solution. The mixture was refluxed at 100 °C
for 8 h with stirring under a dry ice condenser. The solution was
then filtered and stored overnight in a freezer to precipitate the
product. The crude product was filtered, and the resulting solid was
recovered using a rotary evaporator, then recrystallized three times
from ethanol. The final yields were 4.50 g, 10.7 mmol eicosyltrimethylammonium
bromide (C20TAB, 90%, off white solid), 4.22 g, 9.4 mmol
docosyltrimethylammonium bromide (C22TAB, 84%, white solid),
and 0.78 g, 1.6 mmol tetracosyltrimethylammonium bromide (C24TAB, 88%, white solid).
Scheme 1
Reaction of 1-Bromoeicosane with Trimethylamine
to Form the Quaternary
Ammonium Salt, C20TAB; The Same General Method Was Used
for C22TAB and C24TAB Surfactants
The synthesis of 1-bromotetracosane (C24H49Br) was based on the procedure reported by Al-Dulayymi
et al.[30] The method involved dissolving
triphenylphosphine
(0.800 g, 3.05 mmol, 99%, Avocado Research Chemicals Ltd.), tetracosan-1-ol
(1.000 g, 2.82 mmol, >98%, Tokyo Chemical Industry UK Ltd.), and N-bromosuccinimide (1.500 g, 8.43 mmol, 99%, Sigma-Aldrich)
in dry dichloromethane (50 cm3). The reaction mixture was
stirred in a water bath at an ambient temperature for 5 h and then
quenched with water (200 cm3), extracted into dichloromethane
and dried using a rotary evaporator to give a red solid. The product
was then refluxed in petroleum ether (500 cm3) and ethyl
acetate (5 cm3), filtered, and then the filtrate was dried
using a rotary evaporator to give C24H49Br (0.892
g, 2.14 mmol, 89.2% yield). The material (C24H49Br) produced was used directly for the preparation of the C24TAB surfactant. NMR and mass spectra are shown in the Supporting Information (Figures S1–S9).
The synthesis
of vertically ordered silica films under electrochemical control used
the method previously reported by Goux et al.[31] The electrolyte was prepared by mixing C14TAB (0.4801
g, 1.43 mmol), C16TAB (0.4801 g, 1.32 mmol), or C18TAB (0.4801 g, 1.22 mmol) into 0.1 mol dm–3 sodium
nitrate (NaNO3) in 20 cm3 water and 20 cm3 ethanol, then tetraethylorthosilicate (TEOS, 98%, Sigma-Aldrich,
905 μL, 4.08 mmol) was added to the solution. The sol was adjusted
close to pH 3 using 0.2 mol dm–3 HCl and allowed
to hydrolyze for a duration of 2.5 h. The ratio for the surfactant:silica
precursors was kept constant at around 0.3 with each experiment containing
a newly made up sol electrolyte.
C20TAB–C24TAB
The sol
preparation was altered slightly when using C20TAB (0.3000
g, 0.71 mmol), C22TAB (0.3000 g, 0.67 mmol), and C24TAB (0.3000 g, 0.63 mmol) due to surfactant solubility problems
with the standard method described above. The electrolytes were prepared
by mixing 0.1 mol dm–3 NaNO3 in water
(20 cm3) with isopropyl alcohol (20 cm3), adding
TEOS (905 μL, 4.08 mmol), and adjusting the sol pH to between
3 and 3.5 using 0.2 M HCl. The sol was then stirred for 90 min. The
surfactants were added to the sol and allowed to stir for a further
60 min. The C22TAB and C24TAB surfactants did
not dissolve in the solution at 25 °C, so the temperature was
raised to 35 °C, which caused dissolution. The solutions were
then allowed to hydrolyze for 2.5 h. The surfactant:silica ratio was
kept constant at around 0.17 with each experiment using a fresh sol
electrolyte.The electrodeposition process of porous silica
was carried out in a Teflon cell with 2 × 1 cm indium tin oxide
(ITO)-coated glass (surface resistivity of 8–12 Ω–1, Sigma-Aldrich) used as the working electrode. Prior
to deposition the electrodes were washed with water and ethanol and
dried under N2 gas. The working electrode was submerged
vertically into the sol electrolyte, a stainless steel cone was used
as the counter electrode and a silver rod was used as the reference
electrode. A Biologic SP150 potentiostat was used to apply a constant
potential of −1.25 V (vs Ag/Ag+) for a duration
of 20 s. After each experiment the silica films (1 × 1 cm) were
quickly rinsed with water and ethanol, then dried in an oven at 130
°C for 16 h. To remove the surfactants from the pore channels,
the films were then immersed in a solution of 0.2 mol dm–3 HCl (Fischer Scientific) in ethanol for 15 min under gentle stirring.
Film Characterization
Cyclic voltammetry (CV) was used
to determine the accessibility of the pores in the mesoporous silica
films on ITO electrodes using a range of redox probes. The aqueous
probe solution was made up of 0.5 mmol dm–3 ferrocene
methanol (FcMeOH), 5 mmol dm–3 hexaammineruthenium(III)chloride
{[Ru(NH3)6]3+}, or 0.5 mmol dm–3 each of potassium hexacyanoferrate(III) {[Fe(CN)6]3–} and potassium hexacyanoferrate(II)trihydrate
{[Fe(CN)6]4–}, in all cases with a 0.1
mol dm–3 NaNO3 supporting electrolyte.
The working electrode was the ITO coated with mesoporous silica. A
platinum gauze was used as the counter electrode, and the reference
electrode contained Ag/AgCl in a 4 mol dm–3 KCl
solution. The potentiostat used was a Biologic SP150.A Rigaku
Smartlab X-ray diffractometer with a Hypix-3000 2D detector was used
for grazing incidence small angle X-ray scattering (GISAXS) experiments.
The incidence angle for all samples was 0.25°, with measurements
collected as two-dimensional (2D) images or as one-dimensional (1D)
scans collected in-plane with 0.5° incident and 0.228° exit
Soller slits and an angle range of 1–10° 2θ. Scanning
electron microscopy (SEM) images were collected using a Jeol JSM-6500F
operating at an accelerating voltage of 5 kV. To overcome charging,
the porous silica films were coated with a thin layer of gold to improve
the conductivity of the material before obtaining the top view and
cross-sectional scanning electron micrographs. The scanning transmission
electron microscopy images of the mesoporous silica films were recorded
using a JEOL ARM200f double-corrected transmission electron microscope
operated at 200 kV. Transmission electron microscopy (TEM) specimens
were prepared by scraping the silica films off the substrate and suspending
them on lacey carbon films. The TEM specimens were tilted during the
TEM observations to allow for the establishment of an edge-on condition
for some silica flakes (the pores directly facing the electron beam),
when the size and the arrangements of the pores are clearly revealed.Ellipsometric porosimetry (EP) experiments were performed using
a dynamic vapor sorption instrument (Surface Measurement Systems Ltd.,
UK) coupled with a FS-1 multiwavelength ellipsometer (Film Sense,
USA). The toluene vapor sorption experiments were carried out in an
environmental chamber at a fixed temperature of 25 °C, at atmospheric
pressure, and with a solvent partial pressure range between 0 and
95% P/P0. The partial
pressure was maintained using mass-flow controllers operating in the
closed-loop mode using dry air as the carrier gas with a flow rate
of 100 mL/min and the partial pressure was monitored using a speed
of sound sensor. The ellipsometer uses four wavelengths (465, 525,
595, and 635 nm) at an angle of incidence of approximately 65°
to produce the highest possible signal intensity in the detector.
The mesoporous silica films were exposed to toluene in fixed partial
pressure steps ranging between 0.5 and 5% for 20 min per step to reach
equilibrium, and the refractive index was continuously measured to
produce isotherms.
Results and Discussion
The EASA
method was originally devised by Walcarius[25] and has been shown to be a versatile method
to produce highly ordered silica films with the pores vertical to
the substrate. Previously, this process largely used C16TAB as the surfactant, resulting in ∼1.6 nm aligned, hexagonally
ordered pores. Walcarius’ group[31] studied the pore accessibility of EASA films grown with C12TAB to C18TAB using a FcMeOH redox probe. It was concluded
that the increase in current from the CV plots was associated with
increases in the lipophilic surfactant chain length.In the
present study, the deposition of silica films using a series
of cationic surfactants with different alkyl chain lengths, C14–C24, was undertaken with the aim of producing
larger pore diameters than currently available in vertical orientation.[26,32] Deposition potentials above −1.2 V (vs Ag/Ag+)
result in no film formation, whereas below −1.3 V (vs Ag/Ag+), the ITO electrode is damaged by the reduction of indium
oxide. Depositions were carried out at −1.2 V. The range at
which depositions occur is between 10 and 20 s, with thickness values
of 40–150 nm, and with lengthier deposition times resulting
in a loss of uniformity due to surface aggregates.[25,33] The longer alkyl chain lengths increase the hydrophobicity of the
surfactant, so the sol composition and temperature had to be varied
to achieve surfactant solubility and produce highly ordered porous
silica films, including switching the ethanol component of the sol
to isopropyl alcohol. The films were then dried and the surfactant
was removed by solvent extraction as described above.
Structure and
Characterization of Mesoporous Silica Films
The pore ordering
and alignment in mesoporous silica were determined
by GISAXS measurements. The 1D in-plane scattering pattern for the
EASA film grown with C22TAB is shown in Figure , and contains the typical
peaks for mesoporous materials with a hexagonal arrangement of cylindrical
pores in P6mm symmetry, namely the
SBA-15[34,35] and MCM[36,37] families of
materials. The incident beam angle found to provide the most intense
diffraction features was 0.25°, and the 10, 11, 20, and 21 peaks
were observed at 2.00, 3.43, 3.96, and 5.22°, respectively. As
calculated from eqs and 2, the 10 peak position corresponds to
a d-spacing of 4.42 nm and the resulting pore spacing/lattice
parameter, aH is 5.10 nm for the silica
film with C22TAB. This set of peaks was observed for films
produced with the surfactants from C14–C22.
Figure 1
(a) 1D in-plane
GISAXS patterns of EASA films with C22TAB and C24TAB deposited at a potential of −1.25
V (vs Ag/Ag+) for 20 s on ITO electrodes [inset: 2D GISAXS
patterns for (i) C22TAB- and (ii) C24TAB-templated
EASA films] and a (b) plot of pore spacing (nm) of the 10 reflection
and pore diameter (nm) determined using EP as a function of surfactant
chain lengths of EASA films grown with C14TAB,[32] C16TAB, C18TAB, C20TAB, C22TAB, and C24TAB. Furthermore,
the GISAXS data are given in the Supporting Information (Figures S10–S13).
(a) 1D in-plane
GISAXS patterns of EASA films with C22TAB and C24TAB deposited at a potential of −1.25
V (vs Ag/Ag+) for 20 s on ITO electrodes [inset: 2D GISAXS
patterns for (i) C22TAB- and (ii) C24TAB-templated
EASA films] and a (b) plot of pore spacing (nm) of the 10 reflection
and pore diameter (nm) determined using EP as a function of surfactant
chain lengths of EASA films grown with C14TAB,[32] C16TAB, C18TAB, C20TAB, C22TAB, and C24TAB. Furthermore,
the GISAXS data are given in the Supporting Information (Figures S10–S13).In contrast, the addition of C24TAB to the sol electrolyte
resulted in a disordered sol–gel film, as indicated by the
1D in-plane GISAXS data shown in Figure . A clear reduction in the intensity of the
broad 10 diffraction peak and a total loss of the 11 and 20 diffraction
peaks were observed, indicating a significant loss of the hexagonal
order. C22TAB is the maximum chain length with which we
could obtain ordered porosity, and thus this approach allows an expansion
in the pore spacing of ordered films up to 5.10 nm.The 2D GISAXS
image of the EASA film templated with C22TAB is presented
in the insets to Figure i. The four in-plane spots in the horizontal
plane relate to the 10, 11, 20, and 21 reflections discussed in the
previous paragraph. The position of these spots in the horizontal
plane shows that the pores are oriented vertically, that is, perpendicular
to the plane of the substrate. The out-of-plane rings of low intensity
are also observed at similar d-spacings to the in-plane
spots, and these features increase in intensity with longer deposition
times. Goux et al.[31] reported similar GISAXS
features for C16TAB-generated EASA films due to the growth
of silica spheres at the surface with random alignment of the hexagonal
pores. The second inset to Figure ii illustrates the 2D GISAXS data obtained for the
EASA film templated with C24TAB after surfactant removal.
The position of the 10 diffraction spots in the horizontal plane shows
this film also has vertically aligned pores, but the hexagonal domains
have very little longer-range order, which also accounts for the low
intensity of the 1D GISAXS pattern.The pore spacings as a function
of the surfactant chain length
(C14, C16, C18, C20, C22, and C24) for a range of EASA films are also
shown in Figure .
An increase in the pore spacing values upon increasing the surfactant
size is observed. This indicates a likely expansion of the pores,
but does not account for the thickness of the pore walls and hence
is not a direct measure of changing the pore diameter. The pore spacing
values start to plateau at longer chain lengths (C22 and
C24), suggesting that the effect on the pore diameter is
reaching a limit using these linear chain surfactants.The silica
films were soaked in a solution containing 0.2 mol dm–3 HCl in ethanol to remove the surfactant from the
films. The top view SEM images (Figure ) provided evidence that the film was free of microcracks.
The coating thickness of mesoporous silica was found through cross-sectional
imaging to be in the range of 100–150 nm, similar to films
produced with C16TAB under similar conditions.[25] The presence of silica aggregates on the electrode
surface arises from the diffusion of hydroxide ions from the electrode–electrolyte
interface and into the neighboring bulk solution.[38] The diffusion layer thickness was calculated using √πDT (where D is the diffusion coefficient
in which OH– equals to 4 × 10–5 cm2 s–1 and t is the time taken for
the deposition process), and is notably larger (500 μm) than
the thickness of the aligned porous silica films. In this diffusion
layer, silica continues to condense in the bulk solution, resulting
in a layer of silica sphere aggregates on the film’s surface
produced through a conventional Stöber process,[24,38] also visible in Figure . This silica overgrowth can be removed using a sticky tape
(Supporting Information, Figure S14), or
reduced by either optimizing the sol composition, or adjusting the
deposition potential and time. The “Scotch tape test”
can also be used to test film adhesion on the ITO surface, and when
the tape is attached directly onto the surface and peeled off the
silica remains intact.
Figure 2
(a) Top view and (b) cross-sectional SEM image of a C18TAB-templated film deposited at an applied potential of −1.25
V (Ag/Ag+) for 20 s on an ITO electrode.
(a) Top view and (b) cross-sectional SEM image of a C18TAB-templated film deposited at an applied potential of −1.25
V (Ag/Ag+) for 20 s on an ITO electrode.To further investigate the porous silica structure, TEM samples
were prepared using EASA films prepared from C22TAB and
C24TAB. The GISAXS data (Figure ) suggested that the strong hexagonal order
found with C16TAB could be maintained to C22TAB, but that with C24TAB experienced loss of order. The
images in Figure support
this finding because the mesopores in the C22TAB-derived
silica film have vertical orientation with large hexagonal domains
across the film. In contrast, the TEM samples of silica films prepared
with C24TAB retain the mesoporosity but have only very
small regions of hexagonal order in the pore structure. This image
was taken through a flake of silica removed from the film with a scalpel
which sits flat on the TEM grid, and the pores are clearly still vertical,
consistent with the GISAXS data.
Figure 3
TEM images of mesoporous silica films
produced with (a) C22TAB and (b) C24TAB deposited
at −1.25 V (vs Ag/Ag+) for 20 s on ITO electrodes.
TEM images of mesoporous silica films
produced with (a) C22TAB and (b) C24TAB deposited
at −1.25 V (vs Ag/Ag+) for 20 s on ITO electrodes.
Ellipsometric Porosimetry
EP was
used to examine the
silica surface of EASA films grown with different sized surfactants.
EP can provide detailed information about the porosity, sorption isotherms,
and pore size distribution (PSD). Toluene was chosen as the probe
molecule as it primarily interacts with surfaces through nonpolar
dispersion forces.[39,40] Highly polar solvents such as
water are likely to produce large hysteresis because of interaction
with the hydroxyl groups on the pore wall during the adsorption–desorption
process.[41] Changes in refractive index
values were measured during the experiment, which allowed for the
volume fraction of toluene present within the pores to be calculated
using the Lorentz–Lorenz effective medium approximation.[39]In Figure a (inset). the silica films containing C16TAB generated an isotherm with characteristics of Type I (b) and
Type IV (b),[42] with a very minor step noticed
at around 0.09 P/P0 when
exposed to toluene vapor. The initial increase in volume fraction
up to 0.17 P/P0 is in
line with the mixed monolayer–multilayer formation on the surface
of the material and large micropore or small mesopore filling, of
which the latter is most likely dominant. The isotherm, unlike typical
Type I (b) and Type IV (b) shapes, does not fully plateau after the
inflection point at P/P0 = 0.17, almost like a Type II isotherm. This indicates that some
degree of heterogeneous mesoporosity is present, which is supported
by the gradual decrease in the pore volume as the pore diameter increases
past 4–5 nm and is also present in all the other films. The
adsorption–desorption branches present minimal hysteresis,
likely due to the size of the mesopores within the material being
below the critical diameter for capillary condensation to occur. In
comparison, nitrogen systems at −196 °C show hysteresis
in pores wider than ∼4 nm,[42] although
the critical diameter for toluene vapor at ambient conditions has
not been determined quantitatively.
Figure 4
The adsorption–desorption isotherms
of the EASA films grown
with (a) C16TAB, (b) C18TAB, (c) C20TAB, and (d) C22TAB surfactants using toluene. The insets
show the corresponding PSDs.
The adsorption–desorption isotherms
of the EASA films grown
with (a) C16TAB, (b) C18TAB, (c) C20TAB, and (d) C22TAB surfactants using toluene. The insets
show the corresponding PSDs.Films made with C18 (Figure b inset) also produced an isotherm with similar
characteristics to C16, with the step of around 0.10 P/P0 still observed. The curvature
at which the uptake levels off is broader than in the film produced
with the C16 surfactant, a result that can be attributed
to a greater degree of heterogeneous mesoporosity for this particular
sample. There is a low level of hysteresis present in the low partial
pressure region of 0–0.2 P/P0, and the solvent volume fraction does not return to
zero after desorption, likely due to some small quantity of toluene
remaining trapped within the material. A common reason for this to
occur is that the material contains micropores or narrow pore constrictions,
in this case with a size similar to that of the kinetic diameter of
toluene (0.585 nm).[43] Micropores are common
in sol–gel-derived materials due to the formation of the polymer
network in the solution.[44] No hysteresis
was present at medium to high partial pressures, again suggesting
that any mesopores present have a size below the critical hysteresis
diameter.The silica film grown with C20TAB (Figure c inset) exhibited
similar
isotherm characteristics as C16 and C18, but
was found to possess a more noticeable, sharper step of around 0.10 P/P0, as is associated with
Type IV isotherms. The sharper step implies that the transition from
surface adsorption to pore filling is more distinct and points to
C20 having a narrower PSD compared to C16 and
C18, as seen in the PSD plot. This phenomenon is common
in ordered mesoporous silicas, with a study by Calleja et al.[45] observing a similar isotherm shape for MCM-41
exposed to nitrogen, although the transition from the surface to pore
filling was observed to occur at a partial pressure of between 0.15
and 0.2 P/P0. There was
also no hysteresis observed at higher partial pressures in the isotherm.For the film with C22TAB, the shape of the isotherm
matches closest to that of a Type IV (a), which is common for mesoporous
materials. The step in the isotherm has shifted toward a higher partial
pressure of around 0.15 P/P0, although still lower than that of typical Type IV (a) isotherms.
It also presents hysteresis, which suggests the critical mesopore
diameter has been reached and capillary condensation occurs. Both
these observations are consequences of an increase in the pore size.
The hysteresis loop most closely matches the H4 and H5 types, which
have been associated with micro–mesoporous carbons and templated
silicas, respectively.[42]Figure also shows
the PSD curves for the silica films with increasing chain lengths,
C16–C22, which were calculated according
to the modified Kelvin equation where the pores are assumed to have
cylindrical symmetry.[34] These plots display
peak pore sizes in the region of 2.82, 3.24, 3.82, and 4.40 nm for
the surfactants of C16–C22, respectively,
which were all obtained from the desorption branch of the isotherms.
The sharp peaks observed in the PSD plots are indicative of a high
level of homogeneous pore size and porosity in the ordered mesoporous
silica materials. Interestingly, it is also noticed that the pore
diameter increases steadily upon increasing the carbon units in the
surfactant chain. This compared well with the GISAXS-derived pore
spacing in Figure , and shows a fairly consistent trend.
Electrochemical Characterization
of the Films
The pore
accessibilities of the mesoporous silica films on ITO electrodes were
investigated using a range of redox active probes: 0.5 mmol dm–3 FcMeOH, 5 mmol dm–3 [Ru(NH3)6]3+/2+, or 0.5 mmol dm–3 [Fe(CN)6]3–/4– in solutions
also containing 0.1 mol dm–3 NaNO3 as
the supporting electrolyte. This approach has been used previously
to demonstrate pore accessibility in EASA-derived silica films,[25,46] with similar results to those shown in the Supporting Information for films made with C18TAB, C20TAB, and C22TAB (Figures S15–S17). The reversible redox reactions of the active species are shown
in eqs –5. In brief, the as-prepared silica films containing
surfactants have a strong suppression of the electrochemical current
with the charged probes, showing the films to have a good coverage.
A smaller reduction in the current and a positive shift in the oxidation/reduction
potentials with ferrocene methanol are due to this probe’s
solubility in the surfactant and interaction with it. After surfactant
removal, the peak potential separations and the currents were similar
to those observed on bare ITO with the ferrocene methanol and ruthenium
hexamine probes. This suggests a high degree of pore accessibility.
In the case of the hexacyanoferrate probe, somewhat lower currents
and a larger peak separation with the mesoporous silica films are
associated with the interaction between the negatively charged silica
walls and the probe, that is, the Gibbs–Donnan effect.[46,47]It is evident that extending
the alkyl
chain length of surfactants (C14, C16, C18, C20, and C22) affects the rate of
diffusion of the active species into the porous silica medium.Figure shows cyclic
voltammograms recorded with a film produced from C22TAB
(others in the Supporting Information,
Figures S18–S21) at scan rates between 2 and 100 mV s–1 for FcMeOH and [Ru(NH3)6]3+/2+ and
between 2 and 20 mV s–1 for [Fe(CN)6]4–/3–. The CV profiles became distorted at faster
scan rates than this for the anionic species. The peak potential (Ip) as a function of the square root of the scan
rate (v1/2) is shown in Figure d–f. A linear trend
is observed for the Ip versus v1/2 scan rate, suggesting that the current is
governed mainly by diffusion.
Figure 5
Cyclic voltammograms of (a) 0.5 mmol dm–3 FcMeOH,
(b) 5 mmol dm–3 [Ru(NH3)6]3+/2+, and (c) 0.5 mmol dm–3 [Fe(CN)6]4–/3– in 0.1 mol dm–3 NaNO3 at various scan rates {2, 5, 10, 20, 50, and 100
mV s–1 for the FcMeOH and [Ru(NH3)6]3+/2+ redox species and 2, 5, 10, and 20 mV s–1 for the [Fe(CN)6]4–/3– redox couple}; the reliance of the peak current as a function of
the square root of scan rates for the film containing C22TAB deposited at −1.25 V (vs Ag/AgCl) for 20 s on an ITO electrode.
All the experiments were carried out after surfactant removal: (d)
FcMeOH, (e) [Ru(NH3)6]3+/2+, and
(f) [Fe(CN)6]4–/3–.
Cyclic voltammograms of (a) 0.5 mmol dm–3 FcMeOH,
(b) 5 mmol dm–3 [Ru(NH3)6]3+/2+, and (c) 0.5 mmol dm–3 [Fe(CN)6]4–/3– in 0.1 mol dm–3 NaNO3 at various scan rates {2, 5, 10, 20, 50, and 100
mV s–1 for the FcMeOH and [Ru(NH3)6]3+/2+ redox species and 2, 5, 10, and 20 mV s–1 for the [Fe(CN)6]4–/3– redox couple}; the reliance of the peak current as a function of
the square root of scan rates for the film containing C22TAB deposited at −1.25 V (vs Ag/AgCl) for 20 s on an ITO electrode.
All the experiments were carried out after surfactant removal: (d)
FcMeOH, (e) [Ru(NH3)6]3+/2+, and
(f) [Fe(CN)6]4–/3–.For the electron transfer reactions of diffusional processes
in
the solution, it is possible to use the Randles–Sevcik (RS)
equation to determine the diffusion coefficients of redox active species
through the porous silica medium. The RS equation (eq ) describes how the current increases
exponentially with the square root of the scan rate. Ip is the peak current, n is the number
of electrons transferred in the oxidation and reduction reactions, A is the electrode area, D is the diffusion
coefficient, T is the temperature, and C is the concentration of the bulk solution. In the CV plots, the
peak potential separations are greater than 59 mV for the various
redox active species, indicating effects due to iR drop and possible quasi-reversible behavior. The RS equation applies
to electrochemically reversible systems, so the calculated diffusion
coefficients will be affected by resistance effects. The shift in
the peak current position with the scan rate is likely due to the iR drop. In Figure b, the increase in the concentration of the ruthenium species
results in a dramatic increase in the current in comparison to the
lower concentrations of FcMeOH and [Fe(CN)6]4–/3– species, indicating a greater iR drop.The RS equation may
be modified to take account of the proportion
of the surface that is accessible to the electrolyte by including
a porosity parameter as shown in eq .[48] Porosity (φ) values
calculated from the pore spacing and pore diameter measurements were
in the range between 0.1 and 0.7 (Supporting Information, Table S1). The apparent diffusion coefficients calculated from
the gradient of the linear plots, and assuming that the concentrations
of the redox species in the film are the same as those in the bulk
solution, are given in Table together with the surfactants and the active species. The
apparent diffusion coefficient increases with the size of the surfactant
used to produce the film, providing further evidence of pore expansion
and easier electrochemical access through the pores with the surfactant
chain length. However, it is noted that the silica film will change
the concentration of the active species in the pores relative to the
bulk solution to some extent. For example, the D values
for [Fe(CN)6]4–/3– species are
smaller than for the FcMeOH and [Ru(NH3)6]3+/2+ species due to the Gibbs–Donnan effect mentioned
earlier.
Table 1
Apparent Diffusion Coefficients of
EASA Films with Surfactants of Increasing Chain Length Using a Range
of Redox Active Probes
Surfactant
D[FcMeOH] (cm2 s–1)
D[Ru(NH3)6]3+/2+ (cm2 s–1)
D[Fe(CN)6]4–/3– (cm2 s–1)
C14TAB
7.4 × 10–7
6.8 × 10–7
1.4 × 10–7
C16TAB
8.9 × 10–7
8.2 × 10–7
2.8 × 10–7
C18TAB
9.4 × 10–7
1.1 × 10–6
3.8 × 10–7
C20TAB
2.3 × 10–6
2.2 × 10–6
5.2 × 10–7
C22TAB
4.6 × 10–6
3.5 × 10–6
8.6 × 10–7
Conclusions
Ordered mesoporous silica
films with vertically aligned pores were
produced by an electrochemically driven sol–gel method (EASA).
The pore size was increased by extending the alkyl chain length in
the [Me3NR]Br surfactant from 16 to 22 carbons using quaternary
ammonium salts, which resulted in larger pore diameters together with
retained order. The film became disordered when adding C24TAB into the sol electrolyte, indicating a constraint in response
to micelle expansion using linear chained surfactants, although the
vertical pore orientation was retained. The ion diffusion rates using
redox active probes were found to increase with the surfactant size
and hence the pore size. The pore size increased from 2.8 nm using
C16TAB to 4.4 nm using C22TAB. The expanded
pores in these vertically aligned mesoporous silica films may make
them more amenable to use as templates for the electrochemical deposition
of nanowires, and change their behavior in sensing applications.
Scheme 1
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5