Journal of Physics D: Applied Physics
J. Phys. D: Appl. Phys. 51 (2018) 304003 (7pp)

https://doi.org/10.1088/1361-6463/aacacd

Hierarchical self-assembly of colloidal
nanoplatelets driven by evaporation
Ravi Kumar Pujala, M Praveen Kumar and Surajit Dhara1
School of Physics, University of Hyderabad, Hyderabad-500046, India
E-mail: pujalaravikumar@gmail.com and sdsp@uohyd.ernet.in
Received 12 April 2018, revised 26 May 2018
Accepted for publication 6 June 2018
Published 4 July 2018
Abstract

Driven colloids are out of equilibrium systems, which self-organise into diverse complex
structures, and have been one of the major driving themes in materials science. We reportexperimental studies on the evaporation driven self-assembly of colloidal nanoplatelets ofmontmorillonite and laponite. The sol-gel transition is studied using a rheometer and imaging
by confocal laser scanning fluorescence microscope and an electron microscope to probe themicrostructures. Evaporation driven thin films prepared from the bicontinuous gel state show
increasing transparency and decreasing surface roughness with increasing concentration oflaponite. Polarising optical microscope shows a microstructure with string-like birefringent
domains. The scanning electron microscope reveals nacre-like structure of the films anddecreasing inter-layer spacing with increasing concentration of laponite. The composite
films showed superior flame-shielding property compared to the films made of individualcomponents.
Keywords: colloids, self-assembly, thin films(Some figures may appear in colour only in the online journal)

1. Introduction

as an inner shell layer of their armour systems. It also makes
up the outer coating of pearls [13, 14]. There have been
numerous approaches to produce composites with a structure
and mechanical properties similar to those of natural nacre[15]. It has been shown that macroscopic nacre-like composite
materials can be made from freeze casting [16, 17], hot-press
assisted slip casting [18], biomineralization or a bottom-up
approach [19], extrusion and roll compaction, [20] and gelcasting and hot-pressing [21]. Thin nacre-like free-standing
composites are also prepared by layer-by-layer self-assembly[22], biomineralization [23], centrifugation [24] and evaporation [25]. In this context, clay minerals made of anisotropic
particles have emerged as ideal candidates for producing
nacre-like structure [26]. The clay particles are considered as
patchy colloids with directional interactions and exhibit several phases including sol, gel, repulsive and attractive glasses[27–29]. They have been used for preparing highly ordered,transparent films of higher strength and stiffness with the help
of appropriate additives [30].
There are some studies reported on the structural analogues
of nacre using nano-clay platelets [26, 31]. A few studies also
showed the formation of artificial nacre structure from dilute

New functional materials are often produced by promoting theself-assembly and hierarchical structure of colloidal particles.
The geometrical shape, size and interaction of colloidal particles determine the structure and functionality of the new mat­erials [1, 2]. Spherical colloidal particles are known to form avariety of self-assembled structures and in most cases they are
well understood [3]. The use of anisotropic colloidal particles
has received greater attention as they are potential candidates
for creating advanced materials with complex architecture.
Moreover, understanding the self-assembly of the anisotropic
colloids is important both from the fundamental and application points of view [4–10].
The inspiration for designing new materials often comesfrom the architecture of natural and biological materials.
Amongst various natural materials, nacre, also known as
mother-of-pearl, has received a lot of attention mainly due
to its exceptional mechanical properties [11, 12]. Nacre is anorganic–inorganic composite material produced by molluscs
1

Author to whom any correspondence should be addressed.

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Figure 1. Schematic illustration of the preparation of bicontinuousgel (BCG). (a) Fractionated montmorillonite (FMMT) and laponite(L) with composition FMMT1/L0.5. (b) Homogeneous solutionand (c) BCG after 24 h. Transmission electron microscope (TEM)
images of L and FMMT particles are shown on the left.

Figure 3. Viscoelastic behaviour of a BCG (FMMT1/L0.5). (a)Evolution of storage (G ) and loss (G) moduli, complex viscosity(η ∗) and tanδ. The insets show two confocal images taken at sol andgel states. The SEM image in the inset is taken after freeze drying.
(b) Yielding of BCG at different aging times of the same samples.

Figure 2. Phase diagram of the FMMT/L system. There exist
different phases such as sol, BCG, glass and phase separation. The
compositions used in this study are circled with dotted lines.

250 nm with a thickness of about 1 nm. Laponite is a charged
disc-like particle with a diameter of about 25 nm and a thickness of about 1 nm. Both clays have negative and positive
charges distributed on the faces and rims, respectively [34].
Dispersions with different concentrations of clay were prepared by dispersing the desired amount of powder in milliQ
water. Ionic strength was adjusted to 5 × 10−4 M NaCl.
When required, the pH of the dispersions was adjusted using
NaOH, HCl and NH4OH. Since the native clay (MMT) has
a wide size distribution ranging from a few nanometres toseveral micrometres, fractionation was done to obtain the
average size of 250 nm following the procedure described in
our previous studies [33–35]. Binary dispersions of fractionated MMT (FMMT) and L were prepared by mixing individual comp­onents, keeping FMMT constant and varying theconcentration of L, ranging from 0.1 to 3.0 wt.%.
Rheological experiments were performed using a MCR501 rheometer (Anton-Paar) with a cone-plate geometry
having a plate diameter of 50 mm and a cone angle of 0.5°.
Real space images were captured on a confocal laser scanning
fluorescence microscope (Leica SP8 with a 100×, numer­
ical aperture (NA)  =  1.4) using an oil immersion objective.

suspensions of montmorillonite (MMT) and in most cases
some additives were used for making a nacre-like structure[26, 31]. In this paper, we report on the evaporation driven
self-assembly and hierarchical structures of colloidal suspensions of mixed nano-clay platelets of MMT and laponite (L)without any additives. This paper is organised as follows: (i)sections 2 and 3.1 discuss the preparation and rheologicalcharacterisation of bicontinuous gels (BCGs); (ii) section 3.2
discusses the evaporation driven self-assembly of colloids
from the BCG state and (iii) sections 3.2–3.4 discuss the characterisation of self-assembled structures followed by a simple
demonstration of an application.

2. Experimental
The nanoplatelets, laponite-RD (L) and Na-MMT(Cloisite-Na) were graciously gifted by BYK additives andInstruments, Germany. MMT is one of the natural clays witha high aspect ratio. The size of the MMT particle is about

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Figure 4. (a), (b) Schematic diagram showing gel to film transition using slow evaporation of various compositions of fractionated MMT(FMMT) and laponite. (c) Images of several dry thin films kept on a printed paper. (VI) FMMT1/L1, (V) FMMT1/L0.75, (IV) FMMT1/L0.5,(III) FMMT1/L0.25, (II) FMMT1/L0.15, (I) 2 wt.% of FMMT; all at room temperature and 50% relative humidity.

Particle morphology was obtained by a JEOL 2100F transmission electron microscope (TEM). Microstructures of thefreeze-dried gel samples were obtained on a FEI Nova Nano
Lab at 2 kV. To prepare the freeze-dried samples, gels wereaged until the evolution is almost stopped, and then freezedried at  −90 °C under a vacuum (10−6 Pa). Films were prepared by evaporating the solvent in 40 and 100 mm diameter
polystyrene petri-dishes at ambient temper­ature of 25 °C andrelative humidity of 50%. The dishes were partially covered
for slowing down the evaporation and prevent entrainment of
airborne debris. Cross sections were studied after fracturing
the films and the scanning electron microscopy (SEM) experiments were performed on a JEOL field emission microscope
after sputter-coating with 4 nm platinum. A polarising optical
microscope (POM) (Olympus BX-51) was used to study the
crystalline properties of the films.

diagram of the FMMT/L system [32]. Figure 2 shows thecomplete phase diagram of the FMMT/L system. In this study
we have worked in the BCG regime with a varying concentration of L. The selected compositions are marked on the phase
diagram with dotted circles.
First, we measured the rheological properties of a composition (FMMT1/L0.5) to understand the gelation. Figure 3(a)shows the variation of G , G, tanδ and complex viscosity(η ∗) as a function of time. It is observed that G , G and η ∗increase and tanδ decreases with time. The growth of G andG is slow during the initial aging (<100 min.). After about
100 min (=tg), the gelation takes place (cross-over of G andG ). Consequently, tanδ decreases from 1 to a small value(2 × 10−2) and tends to reach a plateau after a long waitingtime. Beyond tg, the complex shear viscosity η ∗ also increases
significantly as expected.
Real space imaging experiments were carried out toexplore the self-assembly of the platelets leading to BCG
from a homogeneous solution phase, using a confocal microscope by labelling the clay platelets with rhodamine 6G. The
process of gelation was visualised and recorded over different
scanning areas. Initially, the dispersions were in the sol stateand showed uniform green colour. But as the aging progressed(beyond 100 min), some darker patches started appearing andthe number of such patches increases beyond the gelation time(inset of figure 3(a)). The microstructure of BCGs was further
investigated on the freeze-dried samples using SEM. Freezedrying was done to prevent drying forces from altering the gel

3. Results and discussion
3.1. Rheological studies

Figure 1(a) shows the TEM images of the FMMT and L particles. The individual components of the composite were dispersed in water with the required proportion. For example,
figure 1(b) shows a composite with 1wt.% of FMMT and
0.5wt% of L (FMMT1/L0.5). Initially, it looks like a homogeneous solution but with aging (100 min) it becomes a gel(figure 1(c)). We have recently reported the detailed phase3

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J. Phys. D: Appl. Phys. 51 (2018) 304003

Figure 6. X-ray diffraction (XRD) profiles of powder (shown in theinset) and thin-films (20 micron) made from different compositions.
The numbers indicate the interplanar spacing.

evaporation. Several dry thin films prepared from various
mixing ratios are shown in figure 4(c). The drying rate wasvaried from 11–20 nm s−1. The use of petri dishes allowed
us to separate the films easily without breaking. The dried
films are kept on printed paper and the images are takenusing a camera. The films made from pure FMMT are opaque(figure 4(c)(I)). The transparency of the films increases withincreasing concentration of L in the composite (figure 4(c)(II)–(VI)). This indicates that the scattering of transmitted
light through the films decreases with increasing concentration of L and this could be due to a reduction of the sizeof the microstructure and hence improved packing of thenanoplatelets. The orientational order of the nanoplatelets
in the films during the drying process is expected because
the dispersions contain sodium ions and during the drying,
the gel concentrations increase, leading to thinner electrical
double layers around the platelets. This causes a weak repulsive force which allows face-to-face arrangement of platelets
through the van der Waals attraction.
Figure 5 shows the SEM micrographs of a cross-section
with increasing concentration of L in 1wt% of FMMT. The
pure FMMT film shows a cross-section with a wavy layered
structure (figure 5(a)). The typical layer spacing in pristine
FMMT is about 300 nm. Addition of a small amount of L to
FMMT reduces the ripples of the layers and also the layer
spacing decreases (figures 5(b)–(e)). The layer spacing isabout 100 nm in figure 5(e). In addition, the surface roughnessof the films also decreases as the concentration of L increases,
and subsequently the films became progressively transparent(figure 4(c)). This could be due to the fact that the smaller
nanoplatelets are acting like an adhesive between the bigger
platelets or because of better exfoliation of FMMT particles
in the mixed solution state compared to the single component
FMMT dispersion (figures 5(b)–(d)) or due to the filling ofvoids produced in the films (within the layers or between thetwo successive layers) by bigger particles during the drying
process. Similar observations were reported on graphene

Figure 5. (a)–(e) SEM images showing cross sections of the driedcomposite films. (f)–(j) Surface morphology of the films.

structure. The SEM image clearly shows a highly porous 3Dinterconnected network made of two interacting anisotropic
platelets (see inset of figure 3(a)).
Aging has a strong effect on the yielding behaviour of the
BCGs. Hence, we performed shear tests to probe the yieldingof a BCG. Figure 3(b) shows the variation of shear stress (τ)
with strain amplitude (γ) at various waiting times. A two-stepyielding behaviour is observed with increasing waiting time.
For example, two small peaks in the stress–strain curve areobserved after the gelation (2 h). After 24 h, the peak heights
are enhanced and one peak at lower γ value is much higher
than the one that occurs at higher γ. Similar observations werereported in many colloidal gel systems. The first yielding (γ1)
is attributed to the breakage of interparticle bonding and thesecond one (γ2 ) is due to the intercluster bond breaking andfragmentation of clusters [36].
3.2. Evaporation induced self-assembly

Figure 4(a) shows a schematic diagram for preparing clay
films from BCGs. They are prepared by a solution casting
method in polystyrene petri dishes 40 mm in diameter where
the sample goes to a film state through the gel phase by slow
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Figure 7. (a), (b) Polarising optical microscope (POM) images of different parts of the film (FMMT1/L0.15), (c) fractured film showing thecross section without polariser. (d) Cross-sectional view with crossed polarisers, (e) drying of the gel in a capillary tube of diameter 1 mm,
showing the birefringence at the air–sample interface. (f) Enlarged view showing the variation of retardation across the interface.

oxide/L thin films, where the addition of L reduces the ripplesof graphene oxide [37].
To gain more insight into the structure of the composite
films, we performed x-ray diffraction (XRD) measurements ofthe films as well as the L and FMMT powders. Figure 6 shows
the XRD profiles of some thin films with various compositions. The layered structure is confirmed by the multiple XRD
peaks. The calculated inter-planar spacing of pure FMMT and
L films are given as 1.27 nm and 1.47 nm respectively. Theinter-planar spacings in the composites are similar to that ofthe pure FMMT. This means that the addition of L does notaffect the inter-planar spacing of FMMT. This suggests thatthe voids produced in the films are filled with the smaller L
particles or better exfoliation of FMMT is responsible for
reduced ripples and higher transparency.
Further we looked at the different parts of the films under aPOM. Around the periphery of the disc-like film we observed
string-like birefringent domains along the radial direction(figure 7(a)). These crystalline domains are extended about
200 μm from the rim towards the centre. On the other hand,in the central region of the film, the crystalline domains are
randomly oriented (figure 7(b)). This suggests that the nanoplatelets are self-assembled during the slow evaporation due tothe interfacial force of the liquid along the radial direction. In
the dried state of the film, these create crystalline domains. In
the central region of the film, the evaporation takes place vertically hence the crystalline domains are orientated randomly.
The POM images of the cross-sections obtained by fracturing

Figure 8. SEM micrographs of the thin films prepared fromthe mixture of FMMT nanoplatelets and silica nanoparticles
of average diameter 200 nm (FMMT1/ Si0.1) at pH  =  8.5.
(a), (c) Cross-sectional view of the fractured film at different
magnifications. (b) Surface of the film. (d) Magnified image
showing the embedded Si nanoparticles within the layers. (e)Schematic diagram showing the embedded Si nanoparticles
within the layers.

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Figure 9. Flame-blocking test of a film of thickness 20 μm (FMMT1/L0.5). (a)–(c) A film is kept between a cotton ball and the flame andthe latter is blocked for 40 min, beyond which it penetrates and burns the cotton. (d)–(f) Optical micrographs of the surface of the films after
different exposure times. (g), (h) Physical appearance of the film before and after the fire test. (i), (j) SEM micrographs of the film before
and after the fire test.

diagram of this structure is shown in figure 8(e). This experiment might encourage further studies on the preparation of
artificial nacre structures by incorporating nanoparticles ofdesired size, shape and functionality.

the films without and with crossed polarisers are shown in figures 7(c) and (d), respectively. It clearly shows birefringent
colour with a nacre-like structure.
We have also conducted the drying experiments in a capillary and observed under the POM (figures 7(e) and (f)). The
birefringence is observed only very close to the sample–air
interface. It indicates that the ordering of the nanoplatelets
starts at the interface. Similar observations were also reported
in other clay systems [38, 39].

3.4. Flame -blocking property of the thin films

There are some studies on the flame-blocking properties of
MMT films with and without additives [31, 40, 41]. Inspiredby this, we studied the flame-shielding ability of the composite
films by placing a small cotton ball, 1 cm above the film (seefigures 9(a)–(c)). The thickness of the film was 20 μm. We
found that the pristine FMMT film failed to block the flame
for more than 10 min whereas the composite films blocked
the flame for more than 35 min and the flame-blocking timeslightly increased with the concentration of L. The physicalappearance of the films before and after burning is shown infigures 9(g) and (h). After the exposure to the fire for s­ everal
minutes, the surface morphology of the films was also studied.
Figures 9(d)–(f) show some optical microscope images exhibiting the microstructure of the films after different exposure
times. Initially, larger domains are formed. As the exposure
time is increased, the larger domains break into smaller

3.3. Effect of spherical silica nanoparticles

We also studied the effect of silica nanoparticles of an average
diameter of 200 nm on the formation of the lamellar structure
in FMMT. We added 0.1wt.% of silica nanoparticles in 1wt.%
of FMMT during the gel preparation. This weak gel wasdried slowly under ambient conditions to prepare thin films.
It is interesting to note that no phase separation was observedin the gels or in the dried film. They are mixed homogeneously, and the presence of silica nanoparticles did not affectthe formation of the layer-structure in the film as shown infigures 8(a)–(d). Silica particles mostly go into the voids
between the films without affecting the layers. A schematic
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J. Phys. D: Appl. Phys. 51 (2018) 304003

sizes. SEM images before and after the burning of the films
are shown in figures 9(i) and (j). A substantial change in the
microstructure of the burnt films is observed compared to theunburnt films.

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4. Conclusion
We have prepared binary mixtures of nano-clays of FMMT
and L without any additives. The mixtures spontaneously
evolve from a sol to a gel state with aging. The slow dryingof the weak gel induces self-assembly of the nanoplatelets
to form very thin layers. Several thin layers are further selfassembled to produce ordered thin films whose structure is similar to nacre, the mother-of-pearl. The L particles mostly go into
the voids created by the FMMT particles, thereby reducing theripples of the layers and the surface roughness. A small concentration of Si nanoparticles does not phase separate from FMMT.
They are arrested within the layers creating a hierarchical structure. The FMMT/L films can block the flame for much longer
than pristine FMMT films, and hence composite films are foundto be a superior flame blocker. It may be mentioned that thedrying rate was critical in the evaporation process, and even aslight increase in the drying rate can affect the self-assemblyand hence the quality of films. This process has an advantage of
producing thin films on any substrate since the starting material
is a gel. We believe that the nacre-like structure consisting ofbinary nanoplatelets and nanoparticles has potential for making
advanced materials with a hierarchical structure and improved
physical properties for various applications.
Acknowledgments
R K P acknowledges financial support from the Departmentof Science and Technology for an INSPIRE Faculty grant(DST/INSPIRE/04/2016/002370). S D acknowledges thefunding from DST and SERB. We acknowledge CTWM
Schneijdenberg, Utrecht University for helping with the SEM
measurements and Alfons van Blaaderen for fruitful discussions
and ideas, and allowing us to use the confocal microscope. S D
gratefully acknowledges the support from the DST (DST/SJF/
PSA-02/2014-2015), and DST-PURSE.
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