Researchers from the Centre for Research on Adaptive Nanostructures and Nano-devices (CRANN) at Trinity College Dublin in Ireland and the University of Oxford in the UK have discovered how to split layered materials to give atom thin nano-sheets. Using these sheets, they have created a range of novel two-dimensional nano-materials possessing chemical and electronic properties that could enable new electronic and energy storage technologies.
Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials Method
1. Nanopowders were bought
Commercially available MoS2, WS2 and BN powders were purchased from Aldrich. In addition, NbSe2 and NiTe2 were purchased from American Elements, MoTe2 and MoSe2 from Cerac Incorporated, TaSe2 from Chemsavers and Bi2Te3 from Absco
2. Nanopowders were put in a low power sonic bath in selected solvents
From previous work on the exfoliation of carbon nanotubes and graphene, it was known that successful solvents tend to have surface tension within a well defined range. We chose a selection of solvents with a wide range of surface tensions but deliberately included a number of solvents known to be good dispersants for nanotubes and graphene. We sonicated each powder (10 ml cylindrical vial, starting concentration 1 mg/ml) in a low power sonic bath (Branson 2510E-MT) for 1 hour. This resulted in dark green dispersions for MoS2 and WS2 and milky white dispersions for BN. The resultant dispersions were centrifuged at 500rpm for 90 mins
3. Optimized disperion by adjusting sonication time and centrifugation rate
It is well known that the quality of nano-material dispersions depends heavily on processing parameters such as sonication time and centrifugation rate. We attempted to improve the concentration of material achieved after centrifugation by partial optimisation of the method of sonication, sonication time and centrifugation rate for dispersions of MoS2 and WS2 in NMP and BN in IPA.
4. Settling and Deposition
The sample for deposition was prepared by ice-cooled sonication of 7.5 mg/ml MoS2 in NMP under a point probe for 1 hour at 38% amplitude. The dispersion was left to settle overnight and centrifuged at 1500 rpm for 45 minutes. The supernatant was decanted by pipette and retained for use. Silicon substrates with thermally grown 300 nm oxide were used for deposition and were cleaned by rinsing with isopropanol (IPA) and blow drying. The MoS2/NMP dispersion (0.36 mg/ml final concentration) was diluted by a factor of 100 in IPA for spray deposition. The silicon substrate was maintained at 90°C and 2 ml of the diluted dispersion was applied using an Evolution Airbrush at a pressure of 1.5 bar.
5. Film and composite formation
In order to form thick films, a large dispersed mass is required. This means preparing large volumes of reasonably high concentration dispersions. To achieve this, we altered the dispersion procedure slightly. Large volume (300 ml, 5 mg/ml) dispersions of MoS2, WS2 (NVP) and BN (IPA) were sonicated for 48 hours in a low power sonic bath (Branson 2510E-MT). These were then centrifuged at 500 rpm for 15 minutes and decanted immediately. The concentration of these dispersions was measured as described above. The required volume was then filtered through a 0.45μm PVDF (polyvinylidene fluoride) filter membrane. For BN, this gave good quality films. However the MoS2 and WS2 films produced in this manner were of very poor quality.
Part of the problem is that when sonicated for 48 hours in a round bottomed flask, NVP becomes very viscous, probably due to sonication induced polymerisation. We note that this does not occur when sonicated in a vial as was done in all other experiments, probably due to more effective standing wave formation in the round bottomed flask. This increased viscosity means that the filtration time is extremely long, resulting in poor quality films. To resolve this issue, the poor quality MoS2 or WS2 films were re-dissolved in 60 mls of a 50:50 DMF:NVP mixture by bath sonication of the coated membrane for 30 mins. These dispersions were then used to prepare good quality films
6. Hybrid films
Free standing hybrids films were prepared from 50:50 by weight MoS2:graphene, WS2:graphene and BN:graphene. To do this MoS2, WS2 and BN films were prepared as described above. In each case the film mass was close to 50 mg.
7. Composite films
Dispersions of MoS2, WS2 and BN were used to prepare films of known mass as described previously. These films were then placed in vials of thermoplastic polyurethane (TPU) dissolved in DMF at 50 mg/ml. The film mass and TPU/DMF volume were coordinated such that the nano-sheet:TPU mass ratios were 5wt% or 20wt%. These dispersions were sonicated for 30 mins to disperse the nano-sheets in the TPU/DMF. The resultant composite dispersions were then poured into Teflon trays and the solvent slowly evaporated. The resultant films were dried at 60C under vacuum for 48hours
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