Janus ultrathin film from multi-level self-assembly at air-water interfaces

Article abstract of DOI:10.1039/c4cc06798c

Ultrathin free-standing Janus films were fabricated at air-water interfaces using azopyridine derivatives and poly(acrylic acid) via multi-level self-assembly on molecular and microscopic scales, which showed distinct asymmetric water wetting abilities on

Full text of DOI:10.1039/c4cc06798c

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DOI: 10.1039/C4CC06798C
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Janus Ultrathin Film from Multi-Level Self-Assembly at Air-
Water Interfaces
Cite this: DOI: 10.1039/x0xx00000x
Hongbin Zhang, a,b Rui Hao,a John K. Jackson,c Mu Chiao*b and Haifeng Yu*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
isotropic and anisotropic underwater superoleophobicity ability.14
Ultrathin free-standing Janus films were fabricated at air-
water interfaces using azopyridine derivatives and
poly(acrylic acid) via multi-level self-assembly on molecular
and microscopic scales, which showed distinct asymmetric
water wetting abilities on different surfaces.
Unlike the liquid-liquid interface approach, only a few reports have
focused on air-liquid interface fabrication of Janus films. Fujii et al.
proposed a synthetic method with potential to produce bulk
quantities of films with softness, asymmetry and regularity.15 They
coated two-dimensional polystyrene particle arrays at the air-water
interface by polymerizing pyrrole from the aqueous phase to form
the soft Janus colloidal crystal film.
Janus materials which possess asymmetric architecture based on two
incompatible sides of different chemistry or morphology, have
emerged as a new division of functional materials over past few
years1 and attracted significant attention rapidly due to their
fascinating properties and diverse potential applications in the fields
such as interface stabilizers,2 biological sensors,3 optical probes,4
Furthermore, self-assembly is an elegant and powerful way to
fabricate materials with novel structure and properties, which is also
an essential feature of many strategies to generate Janus structures
on the nano/micro scale. Using prepared Janus nanoparticles, Sashuk
et al. constructed Janus monolayers at two-phase interfaces including
antireflection coatings5 and drug delivery systems.6 Such air-water interfaces17 and liquid-liquid interfaces.18 However, the
only report on macroscopic Janus film from self-assembled non-
Janus materials reported to date (to our knowledge) is from Ou and
coworkers.16 They reported a silver reduced graphene oxide Janus
film formed through an evaporation-induced self-assembly process
in a Teflon dish, during which the silver (air side) and graphene
oxide (solution side) were separated from their aqueous solution.
Herein, we present a manufacturing process for macroscopic free-
standing Janus ultrathin films obtained at air-water interfaces. The
anisotropic materials can exist in different forms, including Janus
particles7 (spheres, cylinders and discs), fibers8 and films.9 In recent
years, major progress has been achieved in the preparation of
nano/micro-sized Janus materials/particles by self-assembly,
modification with mask protection and phase separation.10 However,
producing Janus materials with larger scale two-dimensional (2D)
structure, especially Janus ultrathin films, remains a challenge and
has not been developed sufficiently.
film formation involves
a multi-level self-assembly process,
Asymmetric surface modification on a 2D platform can be a
practicable way to obtain Janus films. Inspired by the Janus feature
including molecular self-assembly and layer-layer self-assembly.
This is coupled with selective exposure of different exterior
compositions under different environments, resulting in unique
surface morphologies of the films and distinct water wetting ability
on their two surfaces. The film was composed of two kinds of
molecular building blocks. One is a synthetic azopyridine derivative,
p-(dodecyloxy)pridylazobenzene (DPAB), which contains one linear
12 carbon alkyl chain. Detailed information of the synthesis and
characterization of DPAB is described in the ESI.† Azopyridine
derivatives have been recently explored for the fabrication of novel
functional materials through self-assembly arising from their π–π
stacking interactions among the azopyridine chromophores and the
potential ability to conjugate with other materials via hydrogen
bonding (H-bond) or halogen bonding.19,20 The other material is
poly(acrylic acid) (PAA) with molecular weight of 2000, which may
combine with azopyridines by forming hydrogen bonds between
pyridyl groups and carboxylic acid groups.21 We anticipated that the
amphiphilic complex of DPAB-PAA might exhibit different
of lotus leaves, Cheng et al. fabricated
a Janus film with
poly(dimethylsiloxane) and epoxy resin using fresh natural lotus
leaves as bio-template.11 Janus films manufactured using such
methods are usually thick and may need a couple of steps. On the
other hand, being different from nano-Janus particles and Janus films
with large thickness, Janus ultrathin films could be directly produced
at interfaces between two distinct phases, for instance, at liquid-
liquid interfaces. Kulkarni et al. grew Janus porous silica films by
hydrolysis
and
condensation
reactions
between
methyltrimethoxysilane and water at heptane-water interfaces.12 The
films showed anisotropic water wetting ability on their two surfaces.
In a similar study by Rao’s group, various nanocrystalline Janus
films of metal oxides, metal chalcogenides and gold were
successfully prepared at toluene-water interfaces.13 Through electro-
polymerization at the interface of dichloromethane and water, Song
et al. fabricated a Janus polypyrrole film with totally different
morphologies on the surfaces facing different media, which presents
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assembly behaviour at the interface of water and air and probably
form an asymmetric film.
The surface morphology of the resulting film was initially
observed under an optical microscope. The obtained images
including polarizing optical images are shDowOIn: 1i0n.10F3iV9gi/e.CwS4AC3rtCic(0lEe6OS79nI†l8inC)e.
Basically, the surface of the film was uniform but not smooth on
microscale (Fig. S3 a&c, ESI†), and there were tiny birefringent
domains homo-dispersed in the film (Fig. S3 b&d, ESI†), suggesting
a part of the DPAB molecules or the complex were self-assembled
into small ordered structures during the film formation.
A
submicrostructure of the film surface could be distinguished at
higher magnification. Fig. 3 shows the images taken by scanning
electron microscope (SEM). Images (a) to (c) are from the surface of
the air side and (d) to (f) are from that of the water side. Interestingly,
both surfaces consisted of interlaced flakes with lamellar structures.
However, the flakes on air side were relatively distorted and smaller
than those on water air, and partly embedded in a transition layer
which was mostly composed of excess DPAB in the film. The
different morphologies may be ascribed to the different
environmental conditions that the surface of film faced during
manufacture and the different mechanisms of solvent loss
(evaporation to air or diffusion to water). Regardless of the small
morphological differences, the flakes of lamellar structure indicated
that a similar multi-level self-assembly occurred with no dependence
on growing environment (air or water). Section view SEM images of
the Janus film were also obtained (Fig. S4, ESI†), showing that two
rough surfaces are linked by a relatively compact transition layer
with thickness of 10-15 µm.
Fig. 1 Chemical structures of materials and fabrication process of the
Janus film (The scale bars represent 5 mm).
Fig. 2 (a) The Janus film on air-water interface bearing water
droplets on its air-side surface. (b) Water contact angles on the Janus
film surfaces of both sides and their average values. The error bars
represent mean ± standard deviation (n = 5).
To further examine the self-assembled lamellar structure, X-ray
diffraction (XRD) measurements were performed on the film. Fig. 4
shows the XRD profiles of DPAB and the composite film. In the
wide-angle region (Fig. 4a), the film shows similar diffraction
patterns (with smaller intensity peaks) to DPAB, indicating that any
excess free DPAB which did not form H-bonds with PAA
precipitated in the film partially in the crystalline form similar to the
starting DPAB material.18 The sharp peaks at 2θ ≈ 1.09° (d = 8.09
nm) and wide peaks 2θ ≈ 0.46° (d = 19.11 nm) observed in the
small-angle region (Fig. 4b) imply that the composite film contained
the lamellar structure formed by DPAB itself. However, a broad
diffraction at 2θ of 0.55°-0.62° appeared in the composite film after
deduction of the effect of the excess DPAB (insert of Fig. 4b). The
presence of this peak may indicate that, as well as the lamella of
DPAB with low spacing, another lamellar structure with a wider
spacing existed in the film. The widest spacing of the lamellar
structure in the film is about 62.21 nm which is probably from the
flakes observed by SEM in Fig. 3.
Figure 1 shows the fabrication process for the ultrathin Janus film
(the thickness is about 12 to 25 µm). Briefly, DPAB (144.0 mg) and
PAA (14.4 mg) were dissolved in tetrahydrofuran (THF, 500 µL)
with a pyridyl to carboxylic acid ratio of approximately 2∶1. After
mixing thoroughly, 10 µL solution was pipetted and dropped onto
the surface of deionized water. When the solvent was gone by
evaporation and diffusion into water, the ultrathin Janus films with
thicknesses of approximately 5 to 8 µm were obtained quickly on the
air-water interface. Solutions composed of mixing ratios of about
1∶1 and 1∶2, and pure DPAB solution were also applied for
comparison, but the film quality was not good and sometimes the
films would not even form. These observations implied that the
component ratio of DPAB and PAA which decides the solution
viscosity and finally the movability of molecules for self-assembly
plays an important role in the film formation.
The unique asymmetric wetting property of the composite Janus
film on air-side and water-side surfaces is shown in Fig. 2. Fig. 2a
shows the water droplets supported by the Janus film formed at the
air-water interface. The average values of water contact angles on
the Janus film surfaces of the air side and water side were measured
to be 143° and 42°, respectively (Fig. 2b). These large differences
between hydrophobic and hydrophilic asymmetric surfaces may
arise from the combined effects of chemical composition and surface
morphology of the Janus films.
Fig. 4 XRD spectra of DPAB and the composite Janus film: (a)
spectra in the wide-angle region, (b) spectra in the small-angle
region.
We considered that the flakes on the Janus film surfaces were
formed by the self-assembly of H-bonded complex DPAB-PAA.
Attenuated total reflection Fourier transform infrared spectroscopy
(ATR-FTIR) was employed to detect the chemical composition on
surfaces of the Janus film (see Fig. S5, ESI†). Both sides of the
Janus film showed similar FTIR spectra with the characteristic
absorbed peaks of DPAB and PAA, indicating PAA was involved in
Fig. 3 SEM images of the Janus film surfaces: (a-c) the surface on
air-side, (d-f) the surface on water-side.
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the construction of flakes on both surfaces of the Janus films. tends to be exposed on aqueous side of the film while DPAB on air-
Moreover, the peak of PAA centred at 2944 cm-1 which represents side, which finally determines the unique asymmetric wetting
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the -OH of the carboxylic acid disappeared in the film and a new property of the Janus film. The fabrication method by self-assembly
peak appeared at 1943 cm-1 on both surfaces. These variations and a at air-water interface is probably scalable to produce other ultrathin
DOI: 10.1039/C4CC06798C
noted shift of the C=O peak of PAA from 1697 cm-1 to 1715 cm-1 free-standing Janus films on macroscopic scale. The result also
after the film was formed imply the formation of H bonds (-O-H…N) reveals a promising way to fabricate multifunctional materials with
between the carboxylic acid of PAA and the pyridyl group of simple building blocks under a pre-designed environment.
DPAB.21,22
Notes and references
a
Department of Materials Science and Engineering, College of
Engineering,
Peking
University,
P.
R.
China.
Email:
yuhaifeng@pku.edu.cn
b Department of Mechanical Engineering, University of British Columbia,
Canada. Email: muchiao@mech.ubc.ca
c Department of Pharmaceutical Sciences, University of British Columbia,
Canada
† Electronic Supplementary Information (ESI) available: The synthesis
and characterization of DPAB, the polarizing optical images and ATR-
FTIR spectra of the Janus film. See DOI: 10.1039/c000000x/
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