A nanostructured Fc(COCH3)2 film prepared using silica monolayer colloidal crystal templates and its electrochromic properties

Article abstract of DOI:10.1039/c7cp05074g

Since oxidation and reduction reactions mainly take place on surfaces, enlarging the specific surface of redox materials is the key to achieving excellent electrochemical performance. In this work, by using silica monolayer colloidal crystal templates (MCCTs), a nanostructured Fc(COCH₃)₂ film is prepared successfully, and such a nanostructure could exhibit the following unique electrochemical properties: the MCCTs could impede the aggregation tendency of Fc(COCH₃)₂ and possess high electrochemical activity; Fc(COCH₃)₂ enlarges the contact area and offers more active sites and faster electronic transmission channels. The structure, optical and electrochemical properties of the nanostructured Fc(COCH₃)₂ were tested and then compared with those of compact Fc(COCH₃)₂ films to evaluate the role of the nanoarchitecture. The unique structure design of the Fc(COCH₃)₂ film enables outstanding performance, showing a large transmittance change (ΔT) of 37% at 550 nm when switched between 0.5 V and -2.5 V, which is approximately ninefold higher than that of the compact Fc(COCH₃)₂ film (approximately 4%). Response times of coloration and bleaching are found to be only 16.15 s and 5.56 s. Furthermore, the nanostructured Fc(COCH₃)₂ film shows much better cycling stability than the compact one. The results indicate that the nanostructure could significantly improve the electrochemical performance of the Fc(COCH₃)₂ film due to the increase in electrochemical active sites and the enhancement of the D-to-A redox switch of ferrocene.

Full text of DOI:10.1039/c7cp05074g

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A nanostructured Fc(COCH ) film prepared using
3
2
silica monolayer colloidal crystal templates and its
Cite this:DOI: 10.1039/c7cp05074g
electrochromic properties†
a
b
a
b
b
Chunxia Hua, Shuliang Dou, Hongbo Xu, Shuai Hou, Hangchuan Zhang,
b
a
a
b
Panpan Zhang, Yang Gan,
Jiupeng Zhao * and Yao Li
*
Since oxidation and reduction reactions mainly take place on surfaces, enlarging the specific surface of
redox materials is the key to achieving excellent electrochemical performance. In this work, by using
silica monolayer colloidal crystal templates (MCCTs), a nanostructured Fc(COCH
successfully, and such a nanostructure could exhibit the following unique electrochemical properties:
the MCCTs could impede the aggregation tendency of Fc(COCH and possess high electrochemical
activity; Fc(COCH enlarges the contact area and offers more active sites and faster electronic
transmission channels. The structure, optical and electrochemical properties of the nanostructured
Fc(COCH were tested and then compared with those of compact Fc(COCH films to evaluate the
role of the nanoarchitecture. The unique structure design of the Fc(COCH film enables outstanding
performance, showing a large transmittance change (DT) of 37% at 550 nm when switched between
.5 V and ꢀ2.5 V, which is approximately ninefold higher than that of the compact Fc(COCH film
(approximately 4%). Response times of coloration and bleaching are found to be only 16.15 s and 5.56 s.
Furthermore, the nanostructured Fc(COCH film shows much better cycling stability than the compact
one. The results indicate that the nanostructure could significantly improve the electrochemical
performance of the Fc(COCH film due to the increase in electrochemical active sites and the
enhancement of the ‘‘D-to-A’’ redox switch of ferrocene.
3 2
) film is prepared
3 2
)
3 2
)
3
)
2
3 2
)
3 2
)
0
3 2
)
Received 27th July 2017,
Accepted 12th October 2017
3 2
)
DOI: 10.1039/c7cp05074g
3 2
)
rsc.li/pccp
typically consisting of silica or polystyrene with a dense, hexagonal
Introduction
10,11
alignment on a given substrate.
Additionally, the interstitial
Recently, many methods have been used to improve the per- spaces between colloidal nanospheres are modified by infilling
formance of electrochromic (EC) materials, such as doping with these interstitial spaces with metals, semiconductors, insulators
other additives and developing nanostructures. There is a and polymers of all types, thereby enabling the formation of hybrid
12–14
significant amount of interest in EC materials at the nanoscale nanostructured EC materials.
as a result of their many advantageous properties, including In the framework of electrochemical recognition, ferrocene
their increased surface area and enhanced electrochemical has been proved to be a remarkable robust building block for
1
–3
15,16
surface activity.
Therefore, a number of approaches have the construction of redox-responsive electrochemical devices.
been developed to fabricate nanostructured EC materials, such Recently, Manner et al. reported the first nanostructured EC
as chemical vapor deposition, atomic layer deposition, emulsions, materials based on a Fc polymer (polyferrocenylsilane gel)
4–7
17
liquid-droplet surfaces and colloidal crystal templates (CCTs).
and a silica opal composite. Then, they used a complicated
Of all the approaches, the CCT method is an advantageous procedure to develop some fast responsive Fc polymer (poly-
technique, because of its relative simplicity and the possibility of ferrocenylmethylvinysilane (PFMVS)) and polyferrocenyldivinyl-
obtaining a nanostructure having high quality and an ordered silane (PFDVS) inverse gel opal to free the electrolyte flow to
8,9
18
arrangement. Typical CCTs are ordered colloidal nanospheres, optimize the electrochemical properties.
This technique
opens the door to EC materials using FC materials as the raw
material. Unfortunately, synthesis of PFMVS and its derivatives
a
b
School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
50001, Harbin, China. E-mail: jpzhao@hit.edu.cn
Center for Composite Materials and Structure, Harbin Institute of Technology,
50001, Harbin, China. E-mail: yaoli@hit.edu.cn
19,20
is often tedious, dangerous and expensive.
Thus, developing a
1
simple and inexpensive way to fabricate EC materials using a low
molecular weight Fc with controllable morphology is becoming
1
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cp05074g more essential. Furthermore, it has been demonstrated that
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ultrathin films can exist in a variety of novel and unique The product was crystallized from alcohol; m.p. 126–128 1C.
geometries, which make these systems even more applicable Yields refer to analytically pure compounds and were not
to the creation of unique nanomaterials.
optimized. The infrared spectra were recorded using the KBr
0
Here, we employ a novel method for nanostructured 1,1 - pellet method on a Fourier transform infrared (FTIR) spectro-
ꢀ
1
diacetylferrocene (Fc(COCH ) ) (ferrocene derivatives having meter (VERTEX-70, Bruker) in the range of 4000–400 cm
.
3
2
two –COCH
3
groups substituted in the Fc unit) based on silica
monolayer colloidal crystal templates (MCCTs). Then we com- Preparation of the nanostructured Fc(COCH
3 2
) film
pare the structure, optical and electrochemical properties of the
nanostructured and compact Fc(COCH films. This study
demonstrates that rational nanostructuring can dramatically
improve the electrochemical performance of the films even
when fabricated from Fc materials with a relatively low mole-
Our preparation of the nanostructured Fc(COCH3)2 film is
outlined in Scheme 1. First, the indium-doped tin oxide substrates
3 2
)
ꢀ
2
(ITO) (B9 O cm , 1 cm ꢁ 4 cm) and silicon wafers were cleaned
with acetone, ethanol and deionized water in an ultrasonic bath
for 20 min, respectively. Once dried, the cleaned silicon wafer was
placed at the edge of a culture dish, which contained deionized
cular mass. The nanostructured Fc(COCH ) film gives a large
3
2
transmittance change (DT) of 37% at 550 nm, which is approxi-
mately ninefold higher than that of the compact Fc(COCH
film (approximately 4%). Furthermore, the silica nanostructured
Fc(COCH film shows much better cycling stability. Response
times of coloration and bleaching were found to be only 16.15 s
and 5.56 s, owing to its unique nanostructure. Consequently, the
water and sodium dodecyl sulfate (SDS; 10 wt%; H
2
O/SDS =
3 2
)
100 mL/30 mL). Then, the silica suspension (silica/deionized
water/ethanol = 0.1 g/1 mL/1 mL) was lifted off the silicon wafer.
Silica nanospheres would float on the solution due to the surface
tension. After the self-assembly process, the silica nanospheres
would come into the monolayer silica template, and the cleaned
ITO was used to pick up the silica MCCTs. Upon fast drying at
room temperature, the MCCTs were then infiltrated with the
3 2
)
resulting nanostructured Fc(COCH ) film is also expected to
3
2
have promising applications in smart windows, displays, and Li
2
1,22
ion batteries.
3 2
as-prepared 0.09 M Fc(COCH ) /ethanol solution owing to its
2
7
superior cyclability, using a quantitative pipette. The sample
was then placed into a desiccator horizontally and dried at
Experimental
Materials
3
0 1C for 8 hours. Finally, the samples were placed in a vacuum
drying oven before sample characterization by scanning electron
microscopy (SEM), FT-IR, cyclic voltammetry (CV) and electro-
chemical measurements.
Monodispersed silica nanospheres with a diameter of 216 nm
and less than 5% diameter standard deviation were synthesized
by the standard St o¨ ber method which has been described
Electrochemical measurements
2
3
elsewhere. The reagents used for the synthesis of silica spheres
including tetraethyl orthosilicate (TEOS, 98%) and ammonium
hydroxide (28%) are purchased from Sigma-Aldrich. Ferrocene
A conventional three-electrode configuration was constructed
with the Fc(COCH ) film on ITO, a platinum wire, and a silver
3 2
wire serving as the working, the counter, and the reference
electrodes, respectively. The quartz glass window on the wall of
the cell provided convenience for optical characterization. The
solvent/electrolyte for cyclic voltammetry was 1 M propylene
(
Fc, 98%), aluminum chloride (AlCl
3
, Z99.0%), acetyl chloride,
ethanol, Na SO , acetone, and methanol were purchased from
2
4
Sinopharm Chemical Reagent Co., Ltd. The solvents, ethanol,
dichloromethane, and acetyl chloride were dried using an Innovative
Technologies Inc. solvent purification system, collected under
vacuum and stored under a nitrogen atmosphere over 4 Å
carbonate/lithium perchlorate (LiClO , Sigma-Aldrich, Z95%,
4
ACS reagent grade). Spectro-electrochemical properties were
investigatedat room temperature using a CHI-600C electrochemical
workstation (Shanghai Chenhua Instrument Co. Ltd) equipped with
a fiber optic spectrometer (MAYA 2000-Pro, Ocean Optics). The CV
4
molecular sieves. Lithium perchlorate (LiClO , Z95%, ACS
reagent grade) and propylene carbonate were purchased from
Sigma-Aldrich. Ultrapure water was purified by a MilliQ system
to a specific resistivity of B18.2 MO cm.
ꢀ
1
measurements were performed at a scanning rate of 50 mV s
3 2
Synthesis of Fc(COCH ) powder
0
1
,1 -Diacetylferrocene was synthesized by a Friedel–Crafts acetylation
24–26
procedure described in the literature.
out in ambient atmosphere. The solvents were dried and distilled
prior to use. Briefly, to a stirred solution of 33.4 g AlCl (0.25 mol)
and CH Cl (40 mL), a mixture of acetyl chloride (23 mL) and dry
All reactions were carried
3
2
2
CH Cl2 (45 mL) was added dropwise. Then ferrocene (18.6 g)
2
dissolved in dry CH Cl (20 mL) was added in the same manner
2
2
and the reaction mixture was stirred for 2 h. The reaction
mixture was poured into water (200 mL) and extracted with
CH
2
Cl
2
(3 ꢁ 100 mL) and then dried with Na
2 4
SO ; subsequently the
3 2
Scheme 1 Fabrication scheme of the nanostructured Fc(COCH ) film
solution was filtered and the solvent was evaporated to dryness. based on the silica MCCTs.
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between voltages of ꢀ3 V to 0.5 V. For comparison purposes, all Fc(COCH
3
)
2
‘‘whiskers’’ scattered on the surface of the nano-
2
29
films were fabricated with the same thickness, 1 ꢁ 3 cm , on spheres through hydrogen bonds. The interstitial area between
ITO-coated glass.
the silica nanospheres becomes wider due to the loss of van der
3
0
Waals interactions between the nanospheres. The silica/
Fc(COCH3)2 nanocomposites have a morphology with small
Characterization
Fc(COCH
structure, confirming the formation of the opal nanostructured
Fc(COCH film. The corresponding cross-sectional images of
the MCCTs and nanostructured Fc(COCH films are shown in
3 2
) nanoparticles present on the monolayer silica
The morphology of the as-prepared Fc(COCH ) films was char-
3
2
acterized using a scanning electron microscope (SEM), Helios
Nanolab 600i (FEI, America), operating at an accelerating voltage
of 15 kV. Fourier transform infrared (FT-IR, VERTEX-70, Bruker)
3 2
)
3 2
)
ꢀ
1
Fig. S1 (ESI†). The cross-sectional images also confirm the
uniform coverage of Fc(COCH3)2 on the MCCT surface. The
photographs of the MCCTs, compact and nanostructured
Fc(COCH ) are shown in Fig. S2 (ESI†). The morphologies of
spectra of the samples were measured from 4000 to 500 cm . The
melting point of the Fc(COCH powder was recorded using a DSC
010 (TA instruments). The reflectance of the films was measured
3 2
)
2
3
2
using a fiber optic spectrometer (MAYA 2000-Pro, Ocean Optics)
over the wavelength range of 400–1000 nm. The incident angle was
vertical to the surface of the substrate.
the samples also agreed with the results of the SEM images. In
addition, elemental mapping as shown in Fig. S3 (ESI†) indicates
3 2
that Fc(COCH ) was uniformly anchored around the silica
nanospheres.
Fig. 2 shows the FTIR absorption spectra of the MCCTs,
compact Fc(COCH3)2 film and nanostructured Fc(COCH3)2
Results and discussion
The morphologies of these films are characterized via SEM film. The characteristic absorption peaks of the silica nanospheres
ꢀ
1
(
Fig. 1). Fig. 1a shows the highly ordered structure of the at 3422, 1105 and 804, and 953 cm are non-symmetric stretching
MCCTs. Here, the silica nanospheres maintain their shape vibration of O–H groups, non-symmetric and symmetric stretching
during their self-assembly into a closely packed arrangement. vibration of Si–O–Si groups, and stretching vibration of Si–OH
The Fc(COCH
3 2
)
film presents a compact and dense structure groups, respectively (Fig. 2a). The vibrational and deformational
ꢀ
1
(
Fig. 1b). It can be seen that Fc(COCH ) is an aggregation of bands of the ferrocenyl ring at 1052, 825, 680 and 561 cm are
3
2
small particles. This was experimentally confirmed; after the readily observed in nanostructured Fc(COCH
3 2
) (Fig. 2b).
solution was uniformly dropped onto the MCCT surface, the Compared with the spectrum of silica, the nanostructured
2
8
uniform silica/Fc(COCH
process, as the solvent evaporates, the solute (Fc(COCH
particles) is deposited on the surface and the interstitial area but also has strong absorption peaks at 3012 and 1656 cm
of these nanospheres (Fig. 1c). As shown in Fig. 1c, we observe corresponding to the C–H bond in the ferrocenyl groups and
the stretching frequency of the CQO bond of Fc(COCH , and
these peaks shift to a lower energy, appearing at 2926 and
)
3 2
film was formed. During the drying Fc(COCH
3 2
) film not only has the corresponding characteristic
ꢀ1
3 2
)
absorption bands at 3391, 1157 and 855, and 947 cm (Fig. 2c),
ꢀ1
,
3 2
)
ꢀ
1 31,32
ꢀ1
1634 cm
.
The intensity of the CQO band at 1656 cm
ꢀ
1
is weakened and shifted to 1634 cm , indicating that the
silica and Fc(COCH3)2 are not simply blended, and complex
interactions exist.
3 2
The successfully fabricated nanostructured Fc(COCH ) film
and the corresponding intermediate materials are also evidenced
by the reflectance spectra, as shown in Fig. 3. Reflection
measurements were performed in the visible and near IR
regions to study the interaction of the fabricated samples with
light. Note that the reflectance of the films is low, due to their
3
3
relatively small thickness. The black curve corresponds to the
Fig. 1 SEM images of the silica MCCTs (a), the compact Fc(COCH
b) and the nanostructured Fc(COCH film (c).
3
)
2
film
Fig. 2 FTIR spectra of (a) the silica MCCTs, (b) the compact Fc(COCH
film, and (c) the nanostructured Fc(COCH film.
3 2
)
(
)
3 2
3 2
)
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have unanimously revealed that the generation of an anion
radical, as a consequence of one electron acceptance, is the
3
6,37
obligate first step in its reduction mechanism.
Hicks et al.
have demonstrated that the electron-donating or -withdrawing
character of the substitute could preferentially orient the
mechanism toward the dimerization reaction or hydrogenation,
3
8
respectively. These findings indicate that the nanostructured
Fc(COCH film has a larger active surface area and shorter
3 2
)
Fig. 3 Reflection spectrum of the silica MCCTs (a), the compact
ion diffusion distance, which may intensify the electrochromic
properties. Furthermore, the CV curves of the compact
Fc(COCH
3
)
2
film (b) and the nanostructured Fc(COCH
3
)
2
film (c).
39
Fc(COCH ) film with uneven or uniform surface are shown in
3
2
reflectance spectrum of the bare MCCTs prepared from silica Fig. S5 (ESI†). The CV curves of these two films exhibit a small
nanospheres with a diameter of 216 nm. The compact Fc(COCH
difference. This indicates that the degree of uniformity of the
film shows the typical reflectance spectrum of a multi-layered film films has no effect on the electrochemical properties. It could be
green curve). Owing to the infiltration of Fc(COCH ) , the initial concluded that the nanostructure plays a considerable role in
3 2
)
(
3
2
reflectance peak observed at 430 nm red-shifts to 443 nm, with a enhancing the electrochemical properties of the Fc(COCH
corresponding intensity increase from 11% to 14% (red curve).
In order to investigate the effects of the nanostructure on the
Meanwhile, the reflectance peak observed at 546 nm red-shifts to electrochemical performance, in site visible and NIR transmittance
88 nm, and the intensity increases from 6.8% to 11.4%. This measurements are also studied. Both the compact and nano-
gradual red-shift can be ascribed to the enlargement of the silica structured Fc(COCH films are colored by applying a constant
nanospheres due to the surface absorption of Fc(COCH
nano- voltage of ꢀ2.5 V (vs. Ag/AgCl) for 30 s and bleached at 0.5 V (vs.
particles. The absorption causes the inter-lattice spacing to Ag/AgCl) for 30 s. The transmittance spectra of the compact and
become larger. In addition to the increase in the inter-lattice silica monolayered/Fc(COCH films between the colored and
3 2
) film.
5
)
3 2
3 2
)
3 2
)
spacing, there is a consequent increase in the density of the lattice bleached states are presented in Fig. 5a and b, and the
that parallels an increase in its effective refractive index. In Fig. 3c, corresponding integrated transmittance modulation (DT%) is
the intensity of the reflection maximum shows an initial gradual shown in Fig. 5c, respectively. The DT% between the colored
increase followed by the infiltration of Fc(COCH
the nanostructured Fc(COCH
film may be considered to have 37% at a wavelength of 550 nm, which is approximately nine-
the properties of CCs. The structural color of this film also shows fold higher than that of the compact Fc(COCH film (4%).
an angle dependency (Fig. S4, ESI†). The structure color changes
Furthermore, to assess the temporal spectral response under
3
)
2
. At this point, and bleached states for the nanostructured Fc(COCH
) film is
3 2
3 2
)
)
3 2
from green to blue, that’s to say the structure can change with the alternating potentials, the switching behavior was typically
view change. This indicates that the vivid color of the nano- characterized by monitoring the transmittance at a wavelength
structured Fc(COCH
of the silica MCCTs.
To evaluate the role of the introduced MCCTs, the electro- transmittance–time response of the compact and nanostructured
chemical behavior of the compact and nanostructured Fc(COCH
Fc(COCH films. In this way, the color of the sample could turn
films is investigated by CV in a three-electrode configuration and from light orange to brown. The photographs of the nanostructured
compared, as shown in Fig. 4, respectively. Several findings are Fc(COCH film in the colored and bleached states are shown in
3 2
)
film depends on the ordered arrangement of 550 nm under alternating square potentials between 0.5 V
ꢀ
1
and ꢀ2.5 V, at a scan rate of 50 mV s . Fig. 6a and b shows the
3
)
2
)
3 2
3 2
)
evident: (I) a relatively inert electrochemical behaviour is seen for
the compact Fc(COCH ) film (Fig. 4a). (II) In comparison, in this
3
2
potential range, the obvious difference is that the current intensity
of the nanostructured Fc(COCH film is larger (Fig. 4). (III) The
cyclic voltammogram of the nanostructured Fc(COCH film has
a pair of reversible redox peaks at ꢀ1.36V/ꢀ1.15 V (Fig. 4b).
All the previous electrochemical studies on the reduction of Fc
3 2
)
3 2
)
34,35
Fig. 4 Cyclic voltammograms of the compact Fc(COCH
3
)
2
film (a) and
Fig. 5 Optical transmittance spectra of the compact (a) and the nano-
the nanostructured Fc(COCH
)
3 2
film (b).
structured Fc(COCH
3
)
2
film (b); corresponding transmittance modulations (c).
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found to be only 16.15 s and 5.56 s, owing to the unique
nanostructure. The above results show that the film’s nano-
architecture has a great influence on its electrochromic properties.
It’s widely accepted that the nanostructure of these films could not
only increase their electrochemically active sites, but also enhance
the ‘‘D-to-A’’ redox switch of ferrocene. Potential applications of
these nanostructured materials include optical sensors, due to
their electrically switchable color, and other novel applications,
e.g., a new type of colored smart window or electric display devices,
could be envisaged.
Conflicts of interest
There are no conflicts to declare.
Fig. 6 Switching response curves of the compact (a) and the nanostruc-
3 2
tured Fc(COCH ) film (b); a single coloration-bleaching cycle displaying
the switching response characteristics of the nanostructured Fc(COCH
film (c).
3
)
2
Acknowledgements
We thank the National Natural Science Foundation of China
(
No. 51572058, 51502057), the International Science & Technology
Fig. S6 (ESI†). The nanostructured Fc(COCH ) film exhibits good
cycling stability during potential transitions, while that of the
compact film decreases rapidly (Fig. S7, ESI†). The nanostructured
3 2
Fc(COCH ) film shows better optical modulation. The response
time from one state to another state is of great importance in
electrochromic application. The switching time was defined as the
time required to reach 90% of a material’s full transmittance
3
2
Cooperation Program of China (2013DFR10630, 2015DFE52770),
the National Key Research & Development Program (2016
YFB0303903), the Foundation of Advanced Research (15HT01001)
and the Foundation of Science and Technology on Advanced
Composites in Special Environment Laboratory.
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40,41
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8
9
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