The presented procedure allows for the low temperature wet chemical preparation of carbon nanosheets through the self-assembly of novel amphiphilic molecules as reactive precursors and their subsequent carbonization at room temperature. Typically the preparation of carbon nanostructures requires high temperatures or pressures and a control over the morphology and chemical functionalization of the prepared carbons is difficult to achieve under such conditions. The developed approach solely requires UV irradiation at room temperature to furnish carbon nanosheets with molecularly defined dimensions below two nanometers and lateral dimensions on the order of centimeters.
To achieve this, we prepared molecules that are reactive, carbon-rich siblings of typical fatty acid amphiphiles and are designed to self-assemble at the air-water interface into monolayers. The monolayer features a densely packed array of hexayne moieties and the carbonization of the films yields a material otherwise only obtained at temperatures beyond 800 degrees Celsius. To begin, dissolve 208 milligrams of the protected hexayane in 15 milliliters of dichloromethane in a 100 milliliter Schlenk flask under inert atmosphere.
Then, add 10 milliliters of methanol. Shield the flask from light with aluminum foil and add sodium methanolate. After stirring the resulting mixture for 30 minutes at room temperature, dilute with 15 milliliters of dichloromethane and transfer into a separatory funnel.
Then wash once with a one molar aqueous hydrogen chloride solution, and once with saturated aqueous sodium chloride solution. Transfer the organic phase into an Erlenmeyer flask and dry it over 30 grams of sodium sulfate. After filtering the organic phase, concentrate the filtrate in vacuo.
Purify the crude product by column chromatography to isolate the hexayne amphiphile as a yellow solution. Store the product of this reaction as a dilute solution in dichloromethane to minimize any decomposition. Spread an aliquot of 100 microliters of the dilute stock solution of the hexayne amphiphile on ultrapure water in a Langmuir trough comprising an infrared reflection absorption spectroscopy setup.
Leave the Langmuir trough to equilibrate for 15 minutes allowing the solvent to evaporate. Compress the layer to a surface pressure of one millinewton per meter with a constant compression rate of five angstroms per molecule and minute by reducing the surface area of the Langmuir trough with the barriers. Monitor the surface pressure by means of the surface pressure microbalance with a filter paper Wilhelmy plate and set the barriers of the Langmuir trough so that a surface pressure of one millinewton per meter is maintained.
Record an IR spectrum with p-polarized light at an angle of incidence of 40 degrees. Compress the layer at a constant compression rate of five angstroms per molecule and minute by further reducing the surface area of the Langmuir trough with the barriers so that surface pressures of three, then five, and finally, eight millinewton per meter are obtained as indicated by the surface pressure microbalance. Record spectra with p-polarized light at an angle of incidence of 40 degrees for each of these surface pressures.
Carefully remove the box enclosing the Langmuir trough. Mount the UV lamp to a support stand and place it approximately 50 centimeters away from the water surface, while ensuring that the interface is covered in the cone of UV light. After confirming that the monolayer is still compressed at a surface pressure of eight millinewton per meter, as measured by the surface pressure microbalance, set the barriers of the Langmuir trough so that they are fixed at the current position.
Expose the air-water interface to UV light. Monitor and record the change of the surface pressure by the surface pressure microbalance equipped with a filter paper Wilhelmy plate throughout the course of the irradiation. Stop the irradiation by turning the lamp off after a total of 40 minutes of irradiation.
Enclose the Langmuir trough in the sealed box in order to avoid a contamination of the interface before allowing the setup to equilibrate for 30 minutes. Then, set the barriers of the Langmuir trough to maintain the surface pressure observed by the surface pressure microbalance after irradiation. Record an IR spectrum with p-polarized light at an angle of incidence of 40 degrees.
Install two previously cleaned sapphire substrates with two pairs of tweezers attached to a mechanical arm. Immerse the substrates in the subphase, and thoroughly clean the air-water interface before spreading. Carefully spread an aliquot of 100 microliters of the dilute stock solution of the hexayne amphiphile in DCM chloroform on ultrapure water in the Langmuir trough.
Leave the Langmuir trough to equilibrate for 15 minutes, allowing the solvent to evaporate. Compress the layer to a surface pressure of eight millinetwon per meter with a constant compression rate of five angstroms per molecule and minute by reducing the surface area of the Langmuir trough with the barriers. Monitor the surface pressure by means of the surface pressure microbalance equipped with a filter paper Wilhelmy plate and set the barriers of the Langmuir trough so that a surface pressure of eight millinewton per meter is maintained.
To transfer the non-carbonized monolayer to a sapphire substrate, maintain the monolayer at a surface pressure of eight millinewton per meter and pull the mechanical arm up at a speed of 1.2 millimeters per minute until the first substrate is completely removed from the subphase. The second substrate needs to remain immersed in the subphase. Carefully retrieve the first substrate carrying the non-carbonized layer from the pair of tweezers, store it under protection from light, and employ it for the intended application when needed.
Mount the UV lamp to a support stand and place it approximately 50 centimeters away from the water surface while ensuring that the interface is covered in the cone of the UV light. Ensure that the monolayer is still compressed at a surface pressure of eight millinewton per meter, and fix the position of the barriers. Expose the air-water interface to UV light while monitoring and recording the change in surface pressure as measured by the surface pressure microbalance throughout the course of the irradiation.
After a total of 40 minutes of irradiation, stop the irradiation by turning the lamp off. In order to transfer the carbonized film to a sapphire substrate, set the barriers of the Langmuir trough so that the surface pressure measured after irradiation is maintained. While keeping the surface pressure constant, retract the mechanical arm holding the substrate from the interface at a speed of 1.2 milliliter per minute until the substrate is completely removed from the subphase.
Carefully retrieve the substrate carrying the carbonized layer from the pair of tweezers. The Carbon-13 nuclear magnetic resonance, or NMR spectrum, of the hexayne amphiphile, shows the signals for all 12 sp hybridized carbon atoms. The compression of a layer of the hexayne amphiphile at the air-water interface gives rise to a surface pressure area isotherm showing two phases, with a steep slope separated by a strongly tilted plateau.
Shown here are IRRA spectra of the hexayne amphiphile film compressed to surface pressures between one millinewton per meter and eight millinewton per meter. A comparison of the IRRA spectra of a film of reactive carbon-rich amphiphile before ultraviolet irradiation, displayed in blue, and after ultraviolet irradiation, displayed in red, shows that the bands of the hexaynes completely disappear. Shown here are Brewster angle microscopy experiments with a film of amphiphile at the air-water interface before and after carbonization by UV irradiation.
After UV irradiation, a clear change in the texture of the film is observed and the carbonized films can be ruptured, leaving large islands floating at the air-water interface. A scanning electron microscopy image of a carbon nanosheet after Langmuir-Schaefer transfer to a Holey Carbon TEM grid, taken at the edge of the carbon nanosheets, shows some draping and wrinkles. Once mastered, this technique can be used to prepare carbon nanosheets with a thickness below two nanometers, and extended lateral dimensions that are only limited by the size of the available Langmuir trough.
While attempting this procedure, it's important to carefully clean the air-water interface, and to calibrate the experimental setup prior to any experiments. And don't forget that the hexayne amphiphile is very reactive, and precautions, such as shielding it from light, need to be taken to avoid premature carbonization. After its development, this technique has paved the way for researchers in the fields of surface coatings or packaging to explore carbon nanosheets as protective layers in composites with other materials.
