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High Throughput Continuous Production of Layered Materials Using Compressible Flows (NSF funded)
2D materials have raised immense interest among the scientific community because of their unprecented properties such as high electrical conductivity, light transparency, high mechanical strength and high insulating properties. Although several methods to exfoliate/synthesis the layered materials have been reported, a high throughput-continuous process was still missing. We have shown through our work in Advanced Materials journal that these layered materials can be exfoliated using high shear rate. In our process, we pass compressed He gas into a material chamber and then through a swagelock needle valve. The high shear rate generated in the CD nozzle causes the layers of the materials to exfoliate into thin sheets of 2D nano materials.
After analyzing almost 100 hBN nano sheets in Atomic Force Microscope, we found out that the average length of the nano sheets were 276 nm while having an average thickness of 4.2 nm. This gives an aspect ration of 65. Also, 43% of the exfoliated nano sheets were 10 layers or less.
The Raman Spectra of hexaginal boron nitride shows a considerable amount of red shift (Fig b) which indicates the reduction of layer using Compressible Flow Exfoliation (CFE). The TEM images (Fig d) show very thin layers of exfoliated nano sheets. The diffraction pattern indicates a single crystalline hBN while Graphene and Molybdenum di Sulphide are poly crystalline.
To demonstrate the utility of our ultrafast compressible flow exfoliation method, we considered improving the barrier properties of polyethyelene terephthalate (PET) by reinforcing it with the exfoliated 2D layered nanomaterials. Polyethyelene terephthalate is commonly used for food and beverage packaging where the simultaneous requirements of high optical transparency and limiting oxygen transport have proven to be a technical challenge. PET films were optically transparent (94% transmittance) and remained transparent (>90%) when 0.017 and 0.15 vol% of CFE-BN or bulk-BN powder were added (Fig a). Addition of 0.15 vol% CFE-BN resulted in an improvement in the modulus of PET by 21% to 1072 ± 15 MPa. By contrast, the same amount of bulk-BN to PET resulted in only a 12% improvement in the modulus (993 ± 55 MPa) (Fig b) Similar to the mechanical properties, a low volume content (0.017 vol%) of either the CFE-BN or bulk-BN did not result in any change to PET’s steady-state oxygen permeation rate (OPR), which was 0.27 ± 0.01 cm3 cm m−2 d atm. Adding 0.15 vol% of the compressible flow exfoliated BN resulted in the OPR dropping by 26% to 0.20 ± 0.01 cm3 cm m−2 d atm. Interestingly, the same amount of bulk-BN to PET caused the OPR to increase to 0.37 ± 0.08 cm3 cm m−2 d atm, which would be detrimental to any barrier packaging applications (Fig c)
2. Quantifying defects in Graphene for High Performance Conductive Ink (Ongoing) (NSF funded)
One of the key advantages of our CFE process was decoupling of exfoliation step from dispersion step unlike other Liquid Phase Exfoliation(LPE) methods. This leads to a very stable dispersion of nano materials in the solution. Various solutions have been tried including NMP, DMF and aqueous sodium cholate. The figure shows the absorbance data from a UV ViS which indicates a highly stable suspension of nano sheets in the solution.
. In our study, we found the ratio of D peak to G peak is significantly less in CFE than that of bath sonicated graphene (ID/IG=0.66 for CFE graphene and ID/IG=1.1 for bath sonicated graphene). The bulk graphite powder had ID/IG ratio of 0.62. This increased number of the defect in bath sonication may be attributed to the prolonged sonication time which is responsible for new edge creation.
CF speed increasing
All absorbance data were taken at 750 nm
The exfoliated graphene flakes had a mean length of 260 nm and mean thickness of 5 nm (15 layers). The concentration of the exfoliated graphene was 0.22 mg/mL. Moreover, 33% of the flakes observed had 10 layers or less. The TEM image shows a single crystalline few layer graphene.
