The chemical exfoliation phenomena in layered GaSe-polyaniline composite
© Aksimentyeva et al.; licensee Springer. 2013
Received: 22 October 2012
Accepted: 6 January 2013
Published: 15 January 2013
To elucidate the nature of polyaniline (PANI)-GaSe mutual interaction, we carried out structural studies of nano-GaSe powders encapsulated by PANI, exploiting X-ray diffraction and high resolution transmission electron microscopy (HRTEM). Mechanically and ultrasonically dispersed GaSe crystals were mixed with aniline, which then underwent polymerization. After such treatment, GaSe nanocrystals (as shown by HRTEM) consist of few elementary monolayer sandwiches of hexagonal GaSe structure along the crystallographic axis c with a mean diameter of 9.2 nm. There was a significant expansion of interplanar distances (up to 0.833 nm) for all of the nanocrystals observed compared to 0.796 nm for the single crystals.
Two dimensional (2D) semiconductor nanocrystals fabricated in the plate-like form have been intensely investigated since the invention of single-layer graphene. The majority of binary compounds among them are either metal dichalcogenides (of molybdenum, vanadium, tungsten) or indium and gallium monochalcogenides. Gallium selenide with chemically passive selenium-terminated (11–20) surfaces has been applied as effective optical material for IR[1, 2] and terahertz[3, 4] ranges, termination layers in heterointerface fabrication[5–7], etc. Unique structure properties stand GaSe among materials suitable for production single layer 2D plates, even extracted and isolated from bulk. Although several groups have already succeeded in mechanical-[8, 9], thermal-, and laser-induced GaSe exfoliation, fabrication of free single sheet particles was found to be not an easy task. The properties of those GaSe foils are essentially substrate-dependent in the mechanical procedures, while higher temperature growth is accompanied by rolling of the sheets into more thermodynamically favorable tubular 3D structures. Other successful attempts resulted in synthesis of colloidal single-layered nanoparticles in organic solutions[12–14] further underwent self-organization into more complicated structures[13, 14] and fabricated by aqueous- or alcohol-based ultrasonification of GaSe powders[15, 16]. The main problem in the application of such objects is synthesis and stabilization chemistry to be rather nonreproducible and hardly to be controlled as a rule. That usually results in unwanted electronic processes on surface-passivator boundaries and requires of the strongly binding ligands, preferably unreactive in respect to the ambient. Recent perspective indicates that 2D plate-like nanoparticles (including those of GaSe) are excellent luminescent emitters due to the suppression of the absorption strengths into one electronic state in contrary to the band for a bulk material. Not long ago, we found that the mutual interaction of components in the hybrid composites containing GaSe and conducting polyaniline (PANI) polymer leads to an increased essential conductivity, UV shifting in GaSe luminescence spectra, plate-like particle formation, etc.. The aim of the presented communication is an elucidation of the nature of the above-mentioned phenomena by means of structural studies of micro- (nano-) GaSe powders encapsulated by PANI, exploiting X-ray diffraction (XRD) and high resolution transmission electron microscopy.
Aniline monomer, para-toluene-sulfonic acid, ammonium persulfate ((NH4)2S2O8) as oxidant were purchased from Aldrich Co., St. Louis, USA. Nanodispersed GaSe powder was obtained by mechanical milling of GaSe crystals, followed by ultrasonication in butanol. Both untreated GaSe single crystal plates and dried-in-vacuum GaSe nanopowders were used for the synthesis of hybrid nanocomposites with polyaniline.
Preparation of composites was carried out under conditions of oxidative polymerization of aniline under (NH4)2S2O8 in an aqueous medium in the presence of toluene sulfonic acid (TSA) as a doping and stabilizing agent. The method of obtaining the composite consists of several stages. Originally, the method was performed by dispersing of about 45 to 150 mg GaSe plates (such samples are further called PANI-GaSe sample) or GaSe powder with particle size of 60 to 80 nm (PANI-powdered GaSe sample) in a solution of surfactant 0.12 M TSA using ultrasonication for 30 min. Then, 0.205 g of monomer droplets was injected in the GaSe dispersion with continuous stirring, and after 10 min, the solution was added with 0.005 ml of 0.47 M solution of oxidant (NH4)2S2O8. The process was carried out at T = 293 K for 24 h. Finally, a dark dispersion of composite was isolated in the form of precipitate by centrifuging. For investigations, we took samples with inorganic component with 10 to 12% wt.
For transmission electron microscopy (TEM) and electron dispersive X-ray (EDX) characterization, a small amount of PANI-powdered GaSe sample (due to untransparency of bulk GaSe for electrons, PANI-GaSe sample was not suitable for TEM characterization) was diluted in anhydrous acetone and centrifuged; few drops of supernatant then were spread over a carbon-coated copper grip followed by drying (in a nitrogen atmosphere). That removes the traces of acetone and PANI capsules from GaSe nanocrystals. For X-ray diffraction measurements, GaSe-PANI and PANI-powdered GaSe samples were placed between two plastic slides.
XRD patterns were recorded in transmittance mode on a STOE STADI P spectrometer (STOE & Cie GmbH, Darmstadt, Germany) equipped with copper X-ray tube (the incident beam was passed through a germanium monochromator to produce Kα 1 radiation with a wavelength of 0.154056 nm). EDX and TEM images were obtained using a HR-TEM (Fei Technai G2 F20 S Twin microscope) operated at 300 keV.
Results and discussion
It is well known that gallium monoselenide crystal lattice (Figure2c) consists of tetralayers: Se-Ga-Ga-Se-, bounded by the weak van der Waals forces. The interlayer distance between selenium-terminated sandwiches is approximately 3.25 Å. Due to this, it is possible to diffusively include polymeric chains of polyaniline between layers of Se-Se (the width of aniline molecule is about 2.8 Å in the thickest point of benzene ring). Obviously, polymerization results in much larger spatial hindrance of long PANI molecules when forming crystalline composite structures based on hexagonal GaSe. This changes the diffraction pattern which now does not accurately describe the prevailing model of orientation, creates the additional diffraction reflections, and is clearly elucidated by HRTEM. When utilizing the single-crystal plates, this composite phase is apparently saved, but there is simply hexagonal GaSe in contrary to the sample PANI-powdered GaSe. As it was mentioned earlier[18, 22], powdered (i.e., fractured) GaSe samples exhibit numerous extended defects-cleavage stairs on the surface. The aniline molecules diffuse through them more effectively, filling van der Waals gap of particles (Figure2c). That forms few ML composite particles based on GaSe-PANI compounds. As we have not observed any lattice fringes that exceeded 8.33 Å for (0002) GaSe crystal planes, we conclude that this is a critical parameter of GaSe-PANI composites based on GaSe crystal structure. Further hindrance of PANI in the van der Waals gap unambiguously leads to the formation of free isolated particles. The low-temperature synthesis procedure and the presence of PANI on GaSe edges permit to avoid thermodynamically preferable rolling of plane-like particles into tubular, onion, or belt-like 3D structures.
Few ML gallium selenide-PANI nanoparticles have been synthesized using chemical exfoliation method. They possess highly crystalline structure similar to bulk GaSe, but with essential broadening of interplanar distances. The obtained few-nanometer thick disk-like flakes possess broad diameter distribution with average value of 9.2 nm. These results enlighten new frontiers for the development of optical nanomaterials. They extend the fabrication techniques such as mechanical and thermal procedures, not suitable either for formation of size controlled or plate-like particles and organic syntheses, drastically affected by stabilizing ligands.
OIA is currently the leading researcher of Physical Chemistry Department. PYuD is working as a senior researcher of Inorganic Chemistry Department. VPS and OAB are professor and associate professor, respectively, of Semiconductor Physics Department. All authors are from the Lviv Ivan Franko National University.
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