Mechanically flexible and optically transparent three-dimensional nanofibrous amorphous aerocellulose

Carbohydrate Polymers 149 (2016) 217–223

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Carbohydrate Polymers
journal homepage: www.elsevier.com/locate/carbpol

Mechanically flexible and optically transparent three-dimensional
nanofibrous amorphous aerocellulose
Farouk Ayadi a,∗ , Beatriz Martín-García b,c , Massimo Colombo b , Anatolii Polovitsyn b,d ,
Alice Scarpellini b , Luca Ceseracciu a , Iwan Moreels b,c , Athanassia Athanassiou a
a

Smart Materials, Nanophysics, Istituto Italiano di Tecnologia, via Morego, 30, 16163 Genova, Italy
Nanochemistry Department, Istituto Italiano di Tecnologia, via Morego, 30, 16163 Genova, Italy,
c
Graphene Labs, Istituto Italiano di Tecnologia, via Morego, 30, 16163 Genova, Italy,
d
Department of Physics, University of Genoa, via Dodecaneso, 33, 16146 Genova, Italy
b

a r t i c l e

i n f o

Article history:
Received 9 December 2015
Received in revised form 18 April 2016
Accepted 23 April 2016
Available online 28 April 2016
Keywords:

Aerogels
Cellulose
Mechanical properties
Nanofibrous materials
Quantum dots

a b s t r a c t
Aerocelluloses are considered as “third generation” aerogels after the silica and synthetic polymer-based
ones. However, their brittleness and low optical translucency keep quite narrow their fields of applications. Here, both issues are addressed successfully through the fabrication of flexible and mechanically
robust amorphous aerocellulose with high optical transparency, using trifluoroacetic acid as a solvent
and ethanol as a non-solvent. The developed aerocellulose displays a meso-macroporous interconnected
nanofibrous cellulose skeleton with low density and high specific surface area. We demonstrate its high
efficiency as supporting matrix for nanoscale systems by incorporating a variety of colloidal quatum dots,
that provide bright and stable photoluminescence to the flexible aerocellulose host.
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
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1. Introduction
In the past few years, aerogels have drawn increasing attention for different scientific and industrial applications because of
their extremely low bulk density and high specific surface area
(Du, Zhou, Zhang, & Shen, 2013; Fricke & Emmerling, 1998; Pierre
& Pajonk, 2002). In particular, cellulose aerogels (aerocelluloses)
have somewhat similar structural properties to silica and synthetic polymer counterparts, but also have the major advantage
of being bio-based (Demilecamps, Beauger, Hildenbrand, Rigacci,
& Budtova, 2015; Sescousse, Gavillon, & Budtova, 2011b). Several
attempts to prepare cellulose-based aerogels have been reported
(Granstrom et al., 2011). Commonly, the preparation process of the
aerocellulose involves the following three key steps: (i) Solutionsol transition, (ii) Sol-gel transition (gelation), and (iii) Gel-aerogel
transition (drying) (Sescousse, Gavillon, & Budtova, 2011a). All
three steps determine the microstructure of the aerogel and affect
its properties. The first step requires the dissolution of the cellulose in derivatizing (Liebert, 2010) or non-derivatizing solvent

(Sen, Martin, & Argyropoulos, 2013). After that, coagulation of the
cellulose solution using a non-solvent (regeneration) is followed

∗ Corresponding author.
E-mail address: (F. Ayadi).

by solvent exchange and drying under supercritical CO2 (scCO2 ).
However, the aerocelluloses developed with this strategy do not
combine high surface area, strong mechanical properties, superinsulating properties and high transparency in one material (Gavillon
& Budtova, 2008; Sescousse et al., 2011a). Yet, such aerocelluloses could find potential applications as photocatalysts (Shao, Lu,
Zhang, & Pan, 2013) optical sensors, and detectors (Birks et al.,
2010; Katagiri, Ishikawa, Adachi, Fuji, & Ota, 2015). In addition,
several groups (Mohanan, Arachchige, & Brock, 2005; Sorensen,
Strouse, & Stiegman, 2006; Wang, Shao, Bacher, Liebner, & Rosenau,
2013; Zhang, Yang, Bao, Wu, & Wang, 2013) have also proposed
to include quantum dots (QDs) to enhance the functionality of
aerogels for solid-state optical applications such as sensors or
3D displays, by exploiting the size- and composition-tunable QDs
optical properties (Kovalenko et al., 2015). However, wide-spread
applications of silica aerogels have been hindered by their high
production cost and brittle/fragile mechanical properties. Only a
few papers have reported the fabrication of aerocellulose combining high optical transparency and strong mechanical properties.
Recently, cellulose aerogels with a highly porous network were
reported to combine toughness and transparency (Mi, Ma, Yu,
He, & Zhang, 2016). This was achieved by dissolving cotton pulp
in 1-allyl-3-methylimidazolium chloride (AMIMCl) solution followed by coagulation in aqueous solution of AMIMCl (60 wt%). In
another work also, aerocelluloses with high transparency, mechan-

/>0144-8617/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license ( />

218

F. Ayadi et al. / Carbohydrate Polymers 149 (2016) 217–223

ical toughness, and good heat insulation were prepared from liquid
nanocrystalline cellulose (LC-NCell) dispersions (Kobayashi, Saito,
& Isogai, 2014). Here, we demonstrate an alternative method for
the fabrication of mechanically robust amorphous aerocelluloses
that show flexibility and high optical transparency. The method is
based on the transformation of microcrystalline cellulose (MCC)
into a highly porous three-dimensionally nanofibrous structure
with large specific surface area by using for the first time trifluoroacetic acid (TFA) as a solvent. We present the optimized
conditions to obtain aerocellulose with high optical transparency,
by means of ethanol addition (as a non-solvent) to cellulose-TFA
mixture. Under these optimized conditions, we show the effect of
cellulose concentration on the physicochemical properties of the
transparent aerocellulose. Finally, we demonstrate its potential use
in optical applications through the incorporation of colloidal QDs.
2. Experimental
2.1. Aerogel preparation
2.1.1. Materials
Microcrystalline cellulose (MCC), ethanol and trifluoroacetic
acid (TFA, 99%) were purchased from Sigma-Aldrich and used as
received. All reagents and solvents used were of analytical grade.
2.1.2. Preparation of amorphous cellulosic aerogels
Cellulose solutions with different concentrations (2.0, 4.0 and
5.6% (w/v)) were prepared by dispersing MCC in TFA. The solutions were maintained at 0 ◦ C for 24 h and then were kept at room
temperature for 10 days until the cellulose completely dissolved.
11.2 mL of ethanol (28% of the TFA volume) was added to 40 mL of
each cellulose solution under constant stirring for 30 min at room

temperature (about 25 ◦ C). The mixture was then poured into a circular mold (10 cm2 ). The mold was sealed and allowed to stand for
24 h. This resulted in the formation of a free-standing and transparent organogel. The volume of the organogel was reduced by less
than 5% compared to its volume in the liquid state (just after the
addition of the ethanol).
The obtained gel was immersed in an ethanol bath (200 mL).
The ethanol was exchanged twice every day for at least four consecutive days in order to eliminate all trifluoroacetyl ester groups
(Fig. S1). After solvent exchange by ethanol the volume of alcogel
was reduced by 5–10%. The obtained alcogel was then dried using
supercritical CO2 (scCO2 ) chambers (Malvern, USA). First the system was pressurized at 50 bar, and 10 ◦ C under CO2 liquid for 5 h
and then the system was pressurized at 80 bar and 37 ◦ C for 1 h. The
chamber was then gradually depressurized at 37 ◦ C for 30 min.
The resulting aerogels were conditioned at room temperature
and 58% RH for one week before analyses.
2.2. Preparation of colloidal quantum dots (QDs)
2.2.1. Materials
Cadmium oxide (CdO, ≥99.99%), selenium (Se, 99.99%), trioctylphosphine oxide (TOPO, ≥90%) and trioctylphosphine (TOP,
≥97%) were obtained from Strem Chemicals; zinc diethyldithiocarbamate (97%), octadecylphosphonic acid (ODPA, 97%), oleic acid
(OA) (90%), toluene (≥99.7%), methanol (≥99.9%), ethanol (≥99.8%,
without additive), chloroform (99.0–99.4%) and tetrachloroethylene (TCE, anhydrous, ≥99%) from Sigma-Aldrich; PbCl2 (99.999%)
and sulfur (S, 99.999%) were purchased from Alfa-Aesar; oleylamine (OlAm, 80%) from Acros Organics.
2.2.2. Synthesis of CdSe QDs
To synthesize CdSe QDs, we use a published protocol by Carbone
et al. (2007) with slight modifications. TOPO (3.0 g), ODPA (0.28 g)

and CdO (0.06 g) were mixed in a 50 mL flask and heated to about
150 ◦ C under vacuum for 1 h. Then the system was purged with
nitrogen and heated to above 300 ◦ C to dissolve the CdO until a
clear and colorless mixture was obtained. At this point, 1.5 g of TOP
was inserted into the flask, the temperature was adjusted to the
required injection temperature, and a Se:TOP solution (0.058 g of

Se with 0.360 g of TOP) was injected. The injection temperature and
the reaction time were modified in order to synthesize CdSe QDs
of different sizes.
2.2.3. Coating of CdSe QDs with ZnS
To coat CdSe QDs with ZnS (shell thickness 1–1.5 monolayers),
we adapted a procedure from Dethlefsen & Døssing, (2011) decomposing zinc diethyldithiocarbamate in an amine solution. Typically,
a QD toluene suspension containing 4 ␮M of core CdSe QDs was
injected in 5 mL of OlAm in a three-necked flask. Then the zinc precursor was added in a range from 0.2–0.9 g, depending on the core
diameter (2.1–5.1 nm), at room temperature under stirring. The
mixture was heated to 200 ◦ C under argon atmosphere and kept
for 30 min to complete the zinc salt decomposition. Afterward, the
mixture was cooled to room temperature. The QDs were purified
3 times by repeated addition of methanol followed by centrifugation to precipitate the QDs. The QDs were finally suspended in
chloroform.
2.2.4. Synthesis of Oleic acid-capped PbS QDs
Oleic acid-capped PbS QDs were prepared using a slight modification of the method described in Moreels et al. (2011) Briefly, for the
synthesis 1 g of PbCl2 and 7.5 mL of OlAm were degassed for 30 min
at 125 ◦ C in a three-neck flack under Argon. Then, the temperature
was adjusted to 120 ◦ C, and 2.25 mL of a 0.3 M OlAm-S stock solution was added. The reaction proceeded for 1 min to achieve the
desired size. After synthesis, the OlAm ligands were replaced by
adding OA to the QD suspension in toluene, followed by precipitation with ethanol and resuspension in toluene. The diameter was
determined from the spectral position of the first absorption peak.
2.3. Preparation of cellulose QD composite aerogels
The aerogels were cut into pieces of 1 cm × 1 cm × 0.1 cm using a
sharp blade and were dipped into QD dispersions in chloroform, at
various QD concentrations (5 and 15 ␮M) for 24 h. The obtained
composites were then immersed in ethanol for further solvent
exchange and subsequently dried with scCO2 .
2.4. Characterization
The density of the obtained aerogels was determined by measuring their weights and dividing them by their volumes.

High-Resolution Scanning Electron Microscopy (HRSEM)
images were acquired on a JEOL JSM 7500FA (Jeol, Tokyo, Japan)
equipped with a cold field emission gun (cold-FEG), operating at
10 kV acceleration voltage. The samples have been carbon coated
with a 10 nm thick film using an Emitech K950X high vacuum
turbo system (Quorum Technologies Ltd, East Sussex—UK).
The specific surface area and pore size distribution were evaluated by nitrogen physisorption measurements carried out at 77 K
in a Quantachrome gas sorption analyzer, model autosorb iQ. The
specific surface areas were calculated using the multi-point BET
(Brunauer–Emmett–Teller) model, considering 6 equally spaced
points in the P/P0 range of 0.05–0.30. The pore size distribution was
evaluated from the desorption branches of isotherms according to
the Barrett–Joyner–Halenda (BJH) method. Prior to measurements,
samples were degassed for 22 h at 50 ◦ C under vacuum.
The aerogels were immersed in water for four days and subsequently analyzed by thermogravimetric analysis (TGA) to quantify


F. Ayadi et al. / Carbohydrate Polymers 149 (2016) 217–223

the fraction of absorbed water. We performed the analysis using a
TGA Q500 analyzer (under N2 , 3 ◦ C/min for heating).
Compression tests were conducted using an Instron dual column
tabletop universal testing System 3365 equipped with a 500 N load
cell. Aerocellulose samples were in the form of disks with dimensions of about 2.0 cm of diameter and 0.5 cm of thickness. For each
formulation (different initial MCC concentrations in TFA (2.0, 4.0
and 5.6 wt.%)) five samples were used for compression tests, and
the results were averaged to obtain a mean value. The compression strain rate was set to 10% min−1 . The compression modulus
was determined from the slope of the initial linear region of the
stress-strain curve. The densification stress was determined as the
intersection of the tangent lines to the stress-strain curve in the

initial elastic and the subsequent plastic regions.
Flexibility of thin samples (thickness t = 0.35 mm) of the aerogel was evaluated through 3-point bending tests by Dynamic
Mechanical analysis (Q800, TA Instruments) on a clamp with a span
L = 12 mm. Displacement of the central pin d was applied with the
rate of 1 mm min−1 until failure of the samples. Flexural strain ␧
and stress were calculated according to the following equations:
ε=

6dt
L2

(1)

And
=

3FL
2bt 2

1+6

d
L

2

− 4 t/L

d/L

(2)

Respectively, with b the sample width and F the applied load.
From the strain at maximum stress, the corresponding radius of
curvature R was calculated as follow:
R=

t
2ε

(3)

A piece of aerocellulose was compressed on a uniaxial press
under 15 ton load (1 GPa pressure). The X-ray diffraction (XRD)
pattern of the obtained sample was recorded on a PANalytical
Empyrean X-ray diffractometer equipped with a 1.8 kW CuK␣
ceramic X-ray tube, PIXcel3D 2 × 2 area detector and operating at
45 kV and 40 mA. The diffraction pattern was collected in air at
room temperature using Parallel-Beam (PB) geometry and symmetric reflection mode.
The thermal conductivities of the aerogels were measured using
an ai-Phase Mobile device at 25 ◦ C and 50% RH.
Transmittance spectra were measured using a JASCO V-670
UV–vis spectrophotometer. Samples were in the form of disks with
dimensions of about 1.0 cm of diameter and about 1 mm of thickness.
The steady-state photoluminescence (PL) emission was measured using an Edinburgh Instruments FLS920 spectrofluorometer.
The PL spectrum was collected exciting the samples at 450 nm. For
the PL measurements on dispersed QDs, the solvents used were
chloroform and TCE for the CdSe/ZnS and PbS QDs, respectively.
3. Results and discussion
TFA is used for the dissolution of biomass (Dong et al., 2009;

Zhao, Holladay, Kwak, & Zhang, 2007). Concretely, it has been
employed to transform edible vegetable and cereal wastes into bioplastics and was recently used to synthesize nanoparticles from
different polysaccharides (Ayadi, Bayer, Marras, & Athanassiou,
2016; Bayer et al., 2014). Herein, MCC was dissolved in TFA,
and subsequently different ethanol volumes were added to the
solution. The volume fractions of ethanol (VFe, defined as the
volume of ethanol (VEth ) divided by the volume of all solvents:
(VFe) = VEth /(VTFA + VEth ) used) were 0.20, 0.27, 0.38 and 0.50. The
mixtures were stirred for 30 min at room temperature (RT), and the

219

outcome is schematically depicted in Fig. S2a (i) When VFe = 0.2,
the solution remains clear and keeps the same viscosity even
after 1 week. (ii) With the increment of the ethanol content
(VFe = 0.27), competition for TFA between cellulose and ethanol
occurs, and an esterification reaction takes place between ethanol
and TFA (Fig. S2c). This reaction slowly produces ethyl trifluoroacetate and water (Gallaher, Gaul, & Schreiner, 1996) and may be
responsible for the change of the polarity in the mixtures. Thus,
cellulose–cellulose interactions are favored instead of celluloseTFA interactions resulting in the formation of a free-standing and
transparent organogel after 24 h of ethanol addition. (iii) As ethanol
content further increases (VFe = 0.38), the interaction strength
between TFA and cellulose weakens. Initially, the cellulose seems
to be completely dissolved in the solution but eventually cellulose
aggregates to form a uniform solid phase at the bottom of the vial.
(iv) When VFe = 0.5, the interaction between TFA and cellulose gets
even weaker, and the initially transparent solution becomes immediately turbid and formes a fragile organogel. The differences in
the cellulose-TFA interactions, for the different volume fractions of
ethanol, are responsible for different arrangements of the intra- and
intermolecular interactions in each solution. A wide range of dispersive (␦D), polar (␦p), and hydrogen-bonding components (␦h)

that determine the Hansen solubility parameters (HSPs) (Lan et al.,
2014) is obtained for the different ternary final solution systems
(Ethanol/TFA/cellulose) (Fig. S2b).
The free-standing and transparent organogels obtained for
VFe = 0.27 (condition ii) were the ones used for the aerocellulose production. The aerocelluloses were obtained by drying the
organogels at a critical point dryer chamber (supercritical CO2 ).
After supercritical drying, the final volume was reduced by less
than 15%. The obtained aerocelluloses were transparent and displayed a slight bluish haze under room light exposure, probably
caused by Rayleigh scattering (Fig. 1a). XRD measurements performed on the aerocelluloses showed only broad features indicative
of a fully amorphous structure without any particular diffraction
angles (Jeziorny & Kepka, 1972) (Fig. 1b).
The aerocellulose presented here is among the very few amorphous ones found in the literature. Usually, the initial crystalline
structure of the cellulose changes in the final aerogels to cellulose
II crystalline structure (Wang, Gong, & Wang, 2014). In fact, in our
procedure cellulose-TFA solutions were maintained at 0 ◦ C for 24 h
before dissolving them at room temperature. Unlike other acid systems, TFA molecules at 0 ◦ C, are mainly present as cyclic dimers
(Zhao et al., 2007). These dimers penetrate into the crystalline
regions of cellulose polymer, transforming crystalline sections into
amorphous. The obtained organogel was washed several times to
eliminate all TFA residues and trifluoroacetyl ester groups from cellulose alcogel (Fig. S1). After scCO2 drying process the aerogel keeps
its amorphous nature without showing any small diffraction angles
that would correspond to cellulose II polymorph structure (Fig. 1b).
Interestingly, aerocellulose displays good flexibility (inset in
Fig. 1c), rarely observed in conventional transparent aerogel materials such as silica (Pierre & Rigacci, 2011). Fig. 2c shows the typical
flexural stress-strain behavior of non-ductile materials: (i) linear
increase in load with increasing strain. (ii) Slight deviation from
linearity, which is most likely due to the viscoelastic deformation
of aerocellulose that indicates low plastic deformation. At larger
displacement, maximum stress is reached (iii) at the onset of sample failure. The strain, and radius of curvature that can be achieved
are defined in correspondence of the maximum stress. The minimum radius of curvature achieved was 4.8 mm for aerocellulose

samples, confirming their good flexibility.
The obtained aerocelluloses were studied for different initial
MCC concentrations in TFA (2.0, 4.0 and 5.6 wt.%). SEM images
of the A-NFCAs aerogels obtained for VFe = 0.27 at different MCC
concentrations (2.0, 4.0 and 5.6 wt.%) (Fig. 3a–f) show the highly


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F. Ayadi et al. / Carbohydrate Polymers 149 (2016) 217–223

Fig. 1. (a) Photo of aerocellulose (4 (w/v)%) that shows good optical transparency (8.4 cm of diameter and 0.3 cm of thickness) after scCO2 drying (b) XRD spectra of compressed
aerocellulose and (c) flexural stress-strain curves of aerocellulose (inset: aerocellulose with good flexibility).

Fig. 2. SEM images of aerocellulose at different magnifications obtained using different initial concentrations of microcrystalline cellulose (a and d) 2.0 wt%, (b and e) 4.0 wt%
and (c and f) 5.6 wt%.

nanoporous structure of the network consisting of disorderly and
dispersive nanometer-sized cellulose nanofibers. As shown in SEM
images, the increase of the cellulose concentration from 2.0 (Fig. 2a
and d) to 5.6 wt% results in substantially thicker nanofibers (Fig. 3c).
The density of the A-NFCAs was found to increase with increasing MCC concentration (Fig. 3a) ranging from 72 to 220 mg/cm3 ,
whereas the porosity was decreasing with increasing MCC concentration ranging from 95.5% to 86.0%. The amount of water absorbed
in A-NFCAs was calculated from the weight loss of the TGA curves
at 150 ◦ C (Fig. 3b). As expected, the water absorption capacity
increases as the porosity of A-NFCA increases. The highest water

absorption of 16.8 (g g−1 ) was measured at a density of 72 mg/cm3
(porosity 95.5%).
The optical transmittance spectra of all A-NFCAs (film thickness

of 1 mm) used in this study are shown in Fig. 3c. In the visible range,
more than 50% of the light is transmitted, whereas at longer wavelengths, towards the IR spectrum, the transmittance converges to a
value of 92%, confirming the high transparency of all samples. The
progressive decrease of the transmittance towards shorter wavelengths is due to Rayleigh scattering, with the strongest decrease
observed for the samples with the highest density (220 mg/cm3 ).


F. Ayadi et al. / Carbohydrate Polymers 149 (2016) 217–223

221

Fig. 3. (a) Density and porosity of aerocelluloses as a function of cellulose concentration; (b) TGA of prepared aerocellulose. Inset: amount of absorbed water in A-NFCA
(gwater /gcellulose ) calculated from the weight loss of the TGA curve at 150 ◦ C; (c) Transmittance spectra of aerocellulose at different concentrations of cellulose, (d) Specific
surface area (SSA) of the aerogel estimated from the isotherms. Inset: Representative Nitrogen adsorption–desorption isotherm of the A-NFCA (2%(w/v).

The representative nitrogen adsorption/desorption isotherm of
A-NFCA samples shown in the inset of Fig. 3d displays a type IV
physisorption isotherm suggesting the presence of mesoporosity in
the sample (Sing, Everett, Haul, 1985). It was found that the specific
surface area (SSA) significantly increases with decreasing cellulose
concentration, and thus, density of A-NFC aerogels (Fig. 3d). The SSA
values of A-NFCA, as calculated by the Brunauer–Emmett–Teller
(BET) method, were 492, 361 and 222 m2 /g for cellulose concentrations of 2.0, 4.0 and 5.6 wt%, respectively. The pore size distribution
was evaluated according to the Barrett–Joyner–Halenda (BJH)
method, applicable to materials that exibit a type H1 hysteresis
loop (Sing et al., 1985). The pore size distribution resulted to be
quite broad for the three cellulose concentration, as evident from
Fig. S3. The mean pore size was centered in the range 20–35 nm for
all the A-NFCA samples, with a tendency towards slightly smaller
pores with increasing cellulose concentration.

Fig. 4a shows the compressive stress-strain curve of A-NFCAs.
Their behavior is typical for porous materials under compression.
In particular, they show a linearly elastic deformation under small
strains (<5%), a collapse regime accompanied by plastic hardening
until ∼25–40% strain, depending on the density, and finally a densification stage where the stress rises steeply with compression
because the nanofibers impinge upon each other. In Fig. 4b, the
dependence of the Young’s modulus and yield stress on the density
of A-NFCA is shown. The A-NFCAs with density 72 and 220 mg/cm3
show Young’s moduli of 26 MPa and 105 MPa, and yield stress of
0.7 MPa and 3.4 MPa, respectively. By fitting the graphs in Fig. 4b, we
obtained the Eqs. (1) and (2) that show the relationship between the
bulk density , compressive yield stress * and Young’s Modulus
M for A-NFCA, respectively.
∗∼
M∼

1.4

1.2

(4)
(5)

Both the compressive yield stress and the Young’s Modulus
increase as bulk density of the A-NFCA increases, with a power
of 1.4 and 1.2, respectively. Although the range of density that
is achievable through our procedure is relatively narrow, the
equations fall close to the power-law relationships for LC-NCell
aerogels (Kobayashi et al., 2014) and for open cells porous solids

(Gibson and Ashley, 1997). In order to compare the mechanical
properties of A-NFCAs with various aerogels from the literature,
the specific compression modulus (SCM) is plotted as a function
of SSA. The A-NFCAs with density 72 mg/cm3 show SCM value
(370 MPa cm3 g−1 ) similar to a chemically crosslinked graphene
oxide aerogel (366 MPa cm3 g−1 ) (Huang, Chen, Zhang, Lu, & Zhan,
2013) and higher than aerocellulose produced from 1-ethyl-3methylimidazolium acetate (EMIMAc) solution (159 MPa cm3 g−1 )
(Sescousse et al., 2011a). It is worth noting that A-NFCAs can be
distinguished from the latter aerogels also by their superior optical transparency and flexibility. Compared with (LC-NCell) aerogels
(Kobayashi et al., 2014), the present A-NFCAs have a higher specific
Young’s modulus while showing a slightly lower specific surface
area (Fig. 5c). The aerocellulose obtained from the aqueous AMIMCl
solution by Mi et al. (2016) have a specific Young’s modulus higher
than 900 MPa cm3 g−1 and a surface area about 227 m2 g−1 (Mi et al.,
2016). In this work, cellulose-AMIMCl solution was immersed into
a regeneration bath containing a mixture of AMIMCl and water for
coagulation. This process is mainly affected by AMIMCl and water
counter-diffusion. On the other hand, our procedure involves slow
acid hydrolysis of cellulose chains by TFA (Morrison & Stewart,
1998) followed by a slow sol-gel process. This slow hydrolysis leads
probably to a decrease of the degree of polymerisation of cellulose.
Thus, A-NFCAs have a lower specific Young’s modulus compared to
the aerocellulose obtained by Mi et al. (2016).
A-NFCAs do not have only unique mechanical properties, but
also very low thermal conductivity (␭). In Fig. 4d the thermal conductivity is plotted as a function of bulk density for A-NFCAs;
showing a decrease with decreasing density. The thermal conductivity falls below that of air (25 mW m−1 K−1 ) at a density of
72 mg cm−3 . The values of A-NFCAs’ thermal conductivities are
lower than those of aerocellulose containing magnesium hydroxide nanoparticles (from 56 to 81 mW m−1 K−1 ) (Han, Zhang, Wu, &
Lu, 2015), and higher than those of cellulose/graphene oxide foams
(15 mW m−1 K−1 ) (Wicklein et al., 2015). The low thermal conductivity displayed by A-NFCA is probably related to its narrow pores

(Knudsen effect) and its amorphous structure (Choy & Greig, 1977;
Qiao, Bolot, Liao, & Coddet, 2013).


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F. Ayadi et al. / Carbohydrate Polymers 149 (2016) 217–223

Fig. 4. (a) Compressive stress-strain curves of aerocellulose at different concentration of cellulose (2.0, 4.0, and 5.6 wt%); (b) Young’s Modulus and compressive yield stress
of aerocellulose as a function of density; (c) Comparison of specific Young’s modulus for A-NFCA ( ), aerocellulose from AMIMCI ( ) (Mi et al., 2016), LC-Ncell ( ) (Kobayashi
et al., 2014), aerocellulose from 3% and 15% cellulose–EMIMAc solution ( ) (Sescousse et al., 2011a), and graphene oxide aerogel ( ) (Huang et al., 2013), as a function of
their specific surface area; (d) Thermal conductivity as a function of bulk density for A-NFC.

Fig. 5. PL emission spectra of the QD suspension(dotted lines) and embedded in the aerogel at different QDs concentrations (solid lines) corresponding to (a) core-shell
CdSe/ZnS QDs (2.1 nm core diameter) and (b) core PbS QDs (5.0 nm diameter). The solvents used for the PL spectra of the QDs suspension were chloroform (a) and
tetrachloroethylene (b), respectively.

The incorporation of CdSe/ZnS and PbS colloidal QDs with different size (from 1.8 to 5.1 nm core diameter) in the A-NFCAs provides
photoluminescence (PL) properties to the aerogels in a broad range
from visible (using CdSe/ZnS) to near infrared (NIR, using PbS)
wavelengths (Fig. 5 and see SI for PL characterization, Fig. S4). We
incorporate the QDs into the aerogel by simply immersing the aerogel in the corresponding QD solution prepared in chloroform. In
fact, QDs that have been uniformly dispersed in chloroform could
diffuse into the pores rapidly. In this fashion, we avoid possible
damage to the QDs, due to the acids or other reagents used during
the organogel formation process, which can promote a decrease
of their PL properties. The samples are then subjected to solvent
exchange with ethanol, before being treated another time with
scCO2 . However these two steps do not affect the PL. From the
analysis of the PL spectra, we observe a slight red shift of the PL

peak position from the QDs in solution to the QDs embedded inside
the aerogel. This shift can be attributed to slight QD aggregation in
the aerocellulose pores and channels, leading to a Förster energy
transfer and relative enhancement of the red side of the PL peak
(Crooker, Hollingsworth, Tretiak, & Klimov, 2002; Kagan, Murray,

& Bawendi, 1996; Wang et al., 2013) in combination with a relative
suppression of the blue side due to enhanced Rayleigh scattering
from the aerogel. More importantly, the PL spectral position can be
tuned by choosing the appropriate QD size and composition, and
the PL intensity of the QD-aerocellulose system can be easily controlled by modifying the amount of QDs embedded (Fig. 5). Finally,
we observed that the PL remains stable during at least one-month
storage under ambient conditions, opening up the pathway toward
various applications (Mohanan et al., 2005; Sorensen et al., 2006;
Wang et al., 2013; Zhang et al., 2013).
4. Conclusions
We report a novel method to fabricate transparent amorphous
nanofibrous aerocelluloses with low density. This process can be
well controlled, and allows to prepare a series of aerogels with different nanofiber diameters, porosities, and mechanical strengths by
regulating the initial concentration of MCC. The robust mechanical
behavior combined with the flexibility allows large deformations
without fracture. The developed aerocelluloses are optically trans-


F. Ayadi et al. / Carbohydrate Polymers 149 (2016) 217–223

parent, and their high porosity yields exceptionally good thermal
insulation, below that of air. The interpenetrating, open pore
network of A-NFC also permits rapid transport of liquid-phase
molecular reactants and nanoscale objects into the aerogel, allowing it to act as a highly porous support for functional inclusions.

We validated this by incorporating a variety of fluorescent QDs.
The resulting PL spectrum and intensity of the QD-aerocellulose
system can be tuned from visible to NIR wavelengths by choosing
the QDs composition, size and concentration.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgement
The authors gratefully acknowledge support from the European Union 7th Framework Programme under grant agreement no.
604391 Graphene Flagship.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at />103.
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