Alignment of MoS2 Nanotubes in a Photopolymerizable Liquid

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Alignment of MoS Nanotubes in a PhotoPolymerizable Liquid-Crystalline Material Blaz Tasic, Ales Mrzel, Miroslav Huskic, Xinzheng Zhang, and Irena Drevenšek-Olenik J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp508412w • Publication Date (Web): 21 Oct 2014 Downloaded from http://pubs.acs.org on October 25, 2014

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The Journal of Physical Chemistry

Alignment of MoS2 Nanotubes in a PhotoPolymerizable Liquid-Crystalline Material Blaž Tašič1, Aleš Mrzel2, Miro Huskič3,4, Xinzheng Zhang5, Irena Drevenšek-Olenik1,2,*

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Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI 1000 Ljubljana, Slovenia 2

3

National Institute for Chemistry, Hajdrihova 19, SI 1001, Ljubljana, Slovenia 4

5

J. Stefan Institute, Jamova 39, SI 1000 Ljubljana, Slovenia

Polymer Technology College, SI2380 Slovenj Gradec, Slovenia

The MOE Key Laboratory of Weak-Light Nonlinear Photonics, TEDA Applied Physics Institute & School of Physics, Nankai University, Hongda street 23, Tianjin 300457, China

ABSTRACT: We investigated the orientational distribution of MoS2 nanotubes incorporated into a commercial photo-reactive liquid crystalline medium. Electron microscopy imaging and Raman spectroscopy measurements show that interaction with the liquid crystalline host induces strong directional alignment of the nanotubes. The obtained alignment is “frozen” into the structure by subsequent photo-polymerization reaction, which on one hand prevents agglomeration and on the other hand produces a solid composite film with controlled orientation of the nanotubes. Analysis of the mechanical properties shows that by adding 0.1 wt% of the nanotubes the elastic modulus of the films is increased by 35%. Our results demonstrate that the

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nanotube alignment approach based on photo-polymerizable liquid crystalline media, which is relatively inefficient for carbon nanotubes, might be much more promising for inorganic nanotubes. KEYWORDS: Nanotubes, Polymers, Liquid Crystals, Composite materials 1. Introduction Efficient exploitation of specific physical properties associated with the one-dimensional nature of rod-like fillers for polymer materials requires their directional alignment in a polymer matrix. During the last decade various alignment approaches were developed, in particular for inclusion of carbon nanotubes (CNTs), and the resulting improvements of mechanical, thermal, electrical and optical properties of the composites were examined.1,2 The alignment process can take place before, during or after polymerization reaction.3 The latter typically proceeds via mechanical stretching of the composite film4, while alignment methods applied before or during the polymerization reaction usually exploit shear and flow-induced effects5 or exposure to external electric or magnetic fields.6,7 In these cases polymerization process provides a subsequent stabilization of the obtained aligned configuration. An interesting attitude to nanotube (NT) alignment is their incorporation into a liquid crystalline host. Liquid crystals (LC) are naturally suited for the alignment purpose, because their interaction with the NT dopants in principle leads to a synergetic spontaneous orientational ordering process.8-10 However, experimental observations show, that well-controlled NT-LC mixtures are quite difficult to be prepared because of difficulties with NT dispersability and dispersion stability.11 If conventional surfactants are added to improve the dispersion properties, the resulting surface anchoring modifications are typically such that they contradict requirements


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for good alignment. One of the possible solutions of this problem is functionalization of the NT surface with hairy block-copolymers, which leads to spontaneous LC ordering of the functionalised nanoparticles in highly concentrated solutions.12 Another alternative is direct doping of the NTs into a liquid crystal polymer.13 However, structural investigations have shown that the morphology of the polymer is considerably modified after doping. Recently, Cervini et al. reported alignment of multiwall CNTs by use of a photoreactive LC mixture. This approach combines spontaneous orientational ordering of the NTs in the LC host with a subsequent stabilisation of the aligned structure via photo-polymerization leading to solid polymer films with aligned NTs.

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To obtain good dispersability the authors combined relatively complex

surface processing of the NTs with a specially designed multi-component photoreactive mixture. In this paper we report on investigation of the alignment effect of a photo-polymerizable LC medium on inorganic MoS2 NTs. A commercially available single-component photoreactive mesogen was used in combination with NTs not exposed to any particular surface treatment. We demonstrate that even such a “rough approach” leads to polymer thin films with good dispersion and excellent alignment of the incorporated NTs. Our results suggest that an alignment approach based on photo-polymerizable LC media, which did not stimulate much follow up activities in the field of CNTs, can possibly be much more promising for inorganic nanotubes.

2. Materials and methods Molybdenum disulphide (MoS2) NTs were selected as dopant material because previous experiments showed that they exhibited good dispersability in the conventional thermotropic LC material 4-Octyl-4’-Cyanobiphenyl (8CB).15 The NTs were synthesised via a two-step method.16


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In the first step Mo6SyIz (8.2≤y+z≤10) (MoSI) nanowires are synthesised via the transport reaction conditions starting directly from the constituent elements.17 By subsequent centrifugation and microfiltration, bundles of the MoSI nanowires with diameters of around 50 nm and lengths up to several micrometers are obtained. In the second step, the MoSI bundles are transformed to the MoS2 NTs via a sulphurization reaction taking place at 700° C in a flow of argon gas containing H2S and H2. After complete iodine removal, the obtained multiwall MoS2 NTs retain the shape of the precursor bundles of the MoSI nanowires.18 Commercially available reactive mesogenic diacrylate monomer (RM257, Merck Ltd.) was used as the LC host19. At room temperature the monomer is in the crystalline phase and it exhibits the nematic LC phase in the temperature range from 70°C to 130°C.20 Powders of RM257 monomers and of MoS2 NTs were mixed together at room temperature and then heated to 80°C. Afterwards, the mixture was removed from the hot bath and homogenized by an ultrasonic homogenizer (CP-750, Cole-Parmer) for several minutes. The sonication parameters (20% amplitude, on/off intervals of 1 s and 2 s, respectively) were chosen such that the mixture remained all time in the nematic phase. This strategy resulted in very good dispersion of the NTs. We found that for NT concentration of 3wt% the nematic phase completely vanished, while for concentration of 0.1wt% the range of the nematic phase (∼60°C) was almost the same as in the pure LC material. Consequently we decided to use the concentration of 0.1wt%. A subsequent SEM analysis revealed that due to sonication the average length of the MoS2 NTs is reduced by a factor of 2 with respect to the initial length. After homogenization, the mixture was placed back into the hot bath (80°C) and 1 wt% of photoinitiator (Irgacure 651, Ciba) was added. Then, the mixture was introduced (via capillary action) into commercial glass cells for LCs with a cell gap of 20 µm (SA, Instec Inc.). The inner surfaces of the cells were coated with a surface alignment


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layer providing homogenous (planar) LC alignment. Filled cells were placed on a hot plate (80°C) and irradiated with UV light (Ultra-Vitalux, 300 W, Osram) for 5 min. The polymerised samples were slowly cooled to room temperature and then the glass plates were removed. Finally we obtained self-standing thin polymer films with good mechanical strength and glossy surface appearance. Quality of dispersion and alignment of the films on the micrometric scale was inspected by polarization optical microscopy (POM) (Optiphot-2 Pol, Nikon) using crossed-polarizers configuration. The nanoscale structure of the materials was analysed by the scanning electron microscope (SEM, Jeol JSM-7600F) equipped with an energy-dispersive spectrometer (EDS), and by the high-resolution transmission electron microscope (HR-TEM, Jeol JEM-2100F, 200 keV). TEM samples were prepared on a Formvar/Carbon coated copper grid, while SEM samples were coated with a carbon surface layer. Raman spectra were measured by a confocal micro-Raman scattering system (NTEGRA Spectra, NT-MDT Co.) operating in back scattering geometry. A He-Ne laser beam (632.8 nm) with incident power of 3.5 mW was used as excitation source. We verified that this laser power did not cause any significant irreversible damage of the sample during the measurements. The beam was linearly polarized and its polarization direction with respect to the sample was regulated by the half-wave plate. The scattered light from a selected sample area with diameter of about 10 µm was collected by an optical microscopy objective (10x, NA=0.25) and then analysed by a spectrometer. The investigated range of the Raman shift was 140-660 cm-1. Static and dynamic mechanical properties of the polymerized films were investigated by TA Instruments DMA Q800 device, using thin film tension clamp. The stress-strain tests were


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performed at 25 °C. Dynamic measurements were performed in the temperature range from −20 to 80°C at an oscillation frequency of 1 Hz and strain amplitude of 5 µm.

3. Results and discussion The properties of multiwall MoS2 NTs prepared by our two-step fabrication method are very similar to the properties of the layered MoS2 crystalline phase down to very small radii.21-23 Figure 1 shows a SEM image of the NTs after the sonication (homogenization) procedure. The diameters are in the range of 20-90 nm and typical lengths are several hundred nm, so the average aspect ratio is significantly above 10. The TEM image in the insert reveals the multiwall nature of the NTs.

Figure1. SEM image of the MoS2 NTs after the sonication procedure. Insert: TEM image of two isolated NTs.

Figure 2 shows POM images of the composite film with 0.1 wt% of NTs. The crosses denote the orientations of the polarizers and the red double-arrowed lines denote the orientation of the nematic director n, which corresponds to the preferential orientation of the RM257 mesogens in the nematic LC phase. The polymerized LC material is optically anisotropic with optical axis


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parallel to n, and this is the case also for the composite films. The homogeneity of the colour of the film observed in two different orientations signifies a very uniform optical retardation allover the film. The isolated bright stripes in the left image and black circular regions in the right image are NT aggregates that were not destroyed by the sonication procedure. The density of the aggregates and the accompanying distortions of the LC director field around the aggregates are much smaller than observed when a usual thermotropic LC material is used as a host.11

Figure 2. POM images of the photo-polymerized film of RM257 doped with 0.1 wt% of MoS2 NTs at two different orientations with respect to the crossed polarizers. Crosses denote orientations of polarizer and analyser. The double-arrowed lines indicate the direction of the nematic director n.

Figure 3(a) shows a SEM image of the polymerized composite film recorded in backscattered electron mode (COMPO). The macroscopic appearance of the film can be seen in the inset. The elemental composition analysis (EDS) revealed that the bright elongated objects observed in the image correspond to MoS2. The orientational distribution of the objects (NTs) was obtained by analysing several SEM images taken from different sample regions. The analysis involved 145 isolated NTs and the result is given in Figure 3(b). From the obtained angular distribution we calculated the in-plane orientational order parameter Sp given as:


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cos2 ,

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(1)

where ϕ is the angle between the average orientation of the NTs in the image plane and the orientation of the specific NT, while brackets denote averaging. The obtained value is Sp =0.97, which implies a very high level of the alignment.

Figure 3. (a) SEM image (electron backscattering mode (COMPO)) of a photo-polymerized film of RM257 doped with 0.1 wt% of MoS2 NTs. Bright elongated objects in (a) correspond to NT bundles. The insert shows a photo of the film held by tweezers. (b) Statistical distribution of NT orientation obtained after analysis of several SEM images resolving altogether 145 NT objects.

To test the level of NT alignment on the macroscopic scale, resonance Raman spectroscopy with polarized He-Ne laser light was performed. The photon energy of the 638.2 nm He-Ne laser line corresponds to the absorption edge of the electronic transitions associated with the excitonic states of the bulk MoS2, which provides resonant enhancement of the Raman signal.24 Figure 4(a) shows the Raman spectrum of dried NT mesh obtained after the sonication process. The spectrum is in good agreement with the spectra for MoS2 NTs and other MoS2 nanoparticles reported in the literature.25-27 Three Raman modes ( , , in combination with a


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longitudinal acoustic mode (LA) are observed. Figure 4(b) shows Raman spectra of the NTs incorporated into a polymerized composite film (obtained after subtraction of continuous photoluminescence spectrum from the polymer) detected for two orthogonal linear polarizations of the incident laser beam. For polarization along the alignment direction of the host LC material, denoted by the nematic director n, the obtained spectrum is very similar to the spectrum of the NT powder. The position of some Raman peaks in Figure 4(b) is slightly different from those in Figure 4(a), however, as this effect was varying from one sample region to the other, no systematic red- or blue-shift can be claimed. For orthogonal polarization the Raman signal is very weak and often cannot be resolved from the background. This observation is related to the so-called “antenna effect” in resonance Raman scattering from MS2 NTs (where M denotes metals such as Mo, W, Nb, etc.),28,29 which means that excitation of the electronic resonance providing amplification of the Raman signal is much stronger for the optical field along the NT axis than orthogonal to the NT axis.30,31 We measured Raman intensities of the peaks for two orthogonal polarizations of the incident beam in 15 different regions of the sample and obtained the ratio I/I⊥ =2.1±0.5. This ratio is connected with the angular distribution of the NTs by the following relationship28  ⊥

! "# !

$% & ! $% "#& !

,

(2)

where ε is the ratio between transversal and longitudinal components of the polarizability tensor. Equation 2 besides the usual in-plane order parameter Sp involves also the fourth order moment of the orientational distribution, so, further analysis is possible only by assuming a specific form of the distribution function. By taking a simple boxcar function centered along the direction of


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the nematic director n and ε=0.16±0.05 as measured for the WS2 NTs, 28 the value Sp=0.6±0.2 was obtained. This value proves a good alignment of the MoS2 NTs in the composite film also on the macroscopic scale. However, Sp=0.6±0.2 is considerably lower than found in the nanoscopic analysis (Figure 3). This is presumably because, in contrast to the SEM analysis, in Raman spectroscopy one cannot avoid to collect also some signal from the aggregate regions, inside which the NTs are not aligned. Another possible reason for the apparently worse alignment resolved from the Raman spectroscopy is the value of ε, which can be larger than assumed in our calculation. To verify NT alignment on the macroscopic scale, alternatively, various x-ray diffraction techniques (SAXS, XRD, WAXD) could also be used32,33,34 . But, as these techniques normally do not provide resonant enhancement of the signal, they are more suitable for higher NT concentrations.

Figure 4. (a) Raman spectrum of dried powder of MoS2 NTs used for doping. The thick gray line on the x-axis denotes the spectral region shown in Figure (b). (b) Raman spectra of the photo-polymerized RM257 doped with 0.1 wt% of MoS2 NTs measured for incident laser beam polarized parallel (thin red line) and perpendicular (thick blue line) to the direction of the nematic director n, respectively.


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The last part of our study focused on probing the effect of NT doping on mechanical properties of the polymerized films. The elastic modulus E of pure RM257 is in the range of 0.11 GPa and exhibits strong dependence on the LC orientational profile established before the photo-polymerization reaction.35 Consequently, a strong anisotropy of both, static and dynamic mechanical properties, is expected also for the composite materials based on RM257. The room temperature stress-strain analysis of our samples (Figure 5(a)) reveals that addition of 0.1 wt% of NTs increases the elastic modulus of the material for small strain (ε < 0.1 %) from about 1.1 GPa to about 1.5 GPa, i.e. for around 35 %. The strain was performed along the direction of the nematic director n. A notable increase can also be resolved in the graph for the storage modulus (Figure 5(b)). The data for the loss modulus (inset of Figure 5(b)) reveal that the glass transition temperatures Tg of the pure RM257 and the composite material are quite similar, which signifies that doping does not significantly perturb the polymer nanostructure.

Figure 5. (a) Room temperature stress-strain curve for 50 µm thick films of pure RM257 (open circles) and of RM257 doped with 0.1 wt% of the MoS2 NTs (half solid circles). The strain was applied along the direction of the nematic director n. The inset in the up-left corner shows a magnified image for the strain range 0 0.1 %), adhesion between the NTs and the polymer matrix is probably broken. So the composite material starts to behave very similar to pure RM257, i.e. the values of dσ/dε became about the same (Figure 5(a)). This problem can be solved by functionalization of the NTs with a surface coating that stimulates their adhesion to the specific polymer host. For instance, this can be achieved by a coating involving photoreactive groups that can inter-link with the polymer network. 4. Conclusions Our results demonstrate that directional alignment based on incorporation into a LC host medium can be successfully applied to MoS2 NTs. By using a photo-reactive LC compound, the obtained alignment can be subsequently “frozen” into the structure via polymerization process, which on one hand prevents agglomeration of the dopant and on the other hand produces a solid composite film with controlled orientation of the nanotubes. Such films exhibit strong anisotropy


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of their physical properties. Our samples were prepared in the planar (in-plane) arrangement of the LC director field, as this arrangement is most convenient for experimental testing. However, due to well-established methods for LC alignment in various configurations, the approach can easily be extended also to other geometries, for instance to the so-called homeotropic arrangement in which the NTs are oriented perpendicular to the film surfaces (vertical alignment). In case of MoS2 NTs such an arrangement is particularly interesting in combination with their field-emission properties,37 which opens up new perspectives for application in light emitting devices. Other potential industrial applications involve all kinds of devices in which strong and controllable anisotropy of mechanical or other selected physical properties plays a distinctive role. Examples of such applications are directionally selective strain-gauge sensors and polarization sensitive optical interconnects. 38,39 A unique advantage of the alignment approach, based on photo-polymerizable LCs, is that it can be applied also in various spatially inhomogeneous arrangements, which is very difficult to be achieved by other methods for NT alignment in polymer composites. A simple example of such a configuration is for instance the twisted configuration, standardly used in LCD devices, in which the in-plane alignment direction is continuously rotated from the lower to the upper surface of the film. But, even more complex configurations, such as for instance periodic director field profiles obtained by microstructured surface alignment layers,40 can possibly be “transferred” into the NT orientational profile. We hence believe that integration of inorganic NTs and other inorganic one-dimensional nanoparticles into polymerizable LC materials is very promising also for development of polymer composites with microstructured functional properties.


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AUTHOR INFORMATION Corresponding Author * Irena Drevensek-Olenik, Faculty of Mathematics and Physics, University of Ljubljana Jadranska 19, and J. Stefan Institute, jamova 39, SI1000 Ljubljana, Slovenia, e -mail: [email protected], Phone: +386 14773647 Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally. Notes The authors declare no competing financial interest.

Acknowledgements We would like to thank Dr. G. Scalia for stimulating discussions. We acknowledge financial support in the framework of the Slovenian research program P1-0192 “Light and Matter”, Slovenian research program P2-0145 “Polymers and polymeric materials with special properties”, and International S&T cooperation program of China (2011DFA52870).

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(3) Goh, P. S.; Ismail, A. F.; Ng, B. C. Directional Alignment of Carbon Nanotubes in Polymer Matrices: Contemporary Approaches and Future Advances. Composites: Part A 2014, 56, 103126. (4) Zamora-Ledezma, C.; Blanc, C.; Anglaret, E. Controlled Alignment of Individual SingleWall Carbon Nanotubes at High Concentrations in Polymer Matrices. J. Phys. Chem. C 2012, 116, 13760-13766. (5) Pradhan, B.; Kohlmeyer, R. R.; Chen, J. Fabrication of In-Plane Aligned Carbon NanotubePolymer Composite Thin Films. Carbon 2010, 48, 217-222. (6) Zhu, Y. -F.; Ma, C.; Zhang, W.; Zhang, R. -P.; Koratkar, N.; Liang, J. Alignment of Multiwalled Carbon Nanotubes in Bulk Epoxy Composites via Electric Field. J. Appl. Phys. 2009, 105, 054319. (7) Walters, D. A. et al. In-Plane-Aligned Membranes of Carbon Nanotubes. Chem. Phys. Lett. 2001, 338, 14-20. (8) Lynch, M. D.; Patrick, D. L. Organizing Carbon Nanotubes with Liquid Crystals. Nano Lett. 2002, 2, 1197-1201. (9) Zakri, C. Carbon Nanotubes and Liquid Crystalline Phases. Liquid Crystals Today 2007, 16, 1-11. (10) Lagerwall, J. P. F.; Scalia, G. Carbon Nanotubes in Liquid Crystals. J. Mater. Chem. 2008, 18, 2890-2898.


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(11) Rajh, D.; Shelestiuk, S.; Mertelj, A.; Mrzel, A.; Umek, P.; Irusta, S.; Zak, A.; DrevensekOlenik, I. Effect of Inorganic 1D Nanoparticles on Electrooptic Properties of 5CB Liquid Crystal. Phys. Status Solidi A 2013, 210, 2328-2334.

(12) Zorn, M.; Meuer, S.; Tahir, M. N.; Khalavka, Y.; Sonnichsen, C.; Tremel, W.; Zentel, R. Liquid Crystalline Phases from Polymer Functionalised Semiconducting Nanorods. J. Mater. Chem. 2008, 18, 3050-3058. (13) Topnani, N.; Hamplova, V.; Kašpar, M.; Novotna, V., Gorecka, E. Synthesis, Characterization and Functionalisation of ZnO and TiO2 Nanostructures: used as Dopants in Liquid Crystal Polymers. Liquid Crystals 2014, 41, 91-100. (14) Cervini, R.; Simon, G. P.; Ginic-Markovic, M.; Matisons, J. G.; Huynh, C.; Hawkins, S. Aligned Silane-Treated MWCNT/Liquid Crystal Polymer Films. Nanotechnology 2008, 19, 175602. (15) Avsec, M.; Mertelj, A.; Drevensek-Olenik, I.; Mrzel, A.; Copic, M. Visco-Elastic Properties of Nematic-MoS2 Nanotubes Mixtures. Mol. Cryst. Liq. Cryst. 2005, 435, 163-172. (16) Remškar, M.; Viršek, M.; Mrzel, A. The MoS2 Nanotube Hybrids. Appl. Phys. Lett. 2009, 95, 133122. (17) Rangus, M.; Remškar, M.; Mrzel, A. Preparation of Vertically Aligned Bundles of Mo6S9−xIx (4.5

Alignment of MoS2 Nanotubes in a Photopolymerizable Liquid

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