Scission of 2D Inorganic Nanosheets via Physical Adsorption on a Nonflat Surface

Two-dimensional (2D) nanosheets exhibit attractive properties, making them promising candidates for different applications. The properties of these nanosheets largely depend on their composition and structure. Among the existing techniques, secondary processing of 2D nanosheets has proved effective for modulating the material properties and fabricating devices. For example, drilling graphene through oxygen plasma treatment can produce semiconducting graphene nanomesh while the scission of the 2D nanosheets can lead to the formation of twisted bilayer 2D materials with fascinating properties.

Nanosheet fragmentation occurs when they undergo ultrasonic treatment or vigorous agitation to yield irregular shape and size. Interestingly, the shape and size of the sectioned nanosheets can be controlled via scission of nanosheets along the crystallographic axis. Different methods, like scanning probe lithography, have been used to cut and shape 2D nanosheets. However, secondary processing methods remain underdeveloped, especially for scission along the crystallographic axis. This often requires a thorough examination of the crystal orientation of the nanosheet deposited on a substrate, which remains a difficult task.

Among the existing 2D nanosheets, titania nanosheet is not only popular but also among the most studied nanosheets. They have a uniform crystallographic thickness and relatively large aspect ratio and can be obtained as colloidal 2D crystals monodispersed in solutions. With the ability to undergo neat monolayer tiling, titania nanosheet has proved to be suitable material for investigating the dissociation of chemical bonds through the action of intermolecular forces on substrates.

Herein, Dr. Nobuyuki Sakai, Dr. Masahiko Suzuki and Dr. Takayoshi Sasaki from National Institute for Materials Science in Japan developed scission of 2D inorganic nanosheets along crystallographic axis via physical adsorption on a nonflat surface. In their approach, titania nanosheets were used to examine the possibility of using intermolecular forces acting between the substrate surface and nanosheets to drive scission of 2D nanosheets along the crystallographic axis. The chemical bonds inside the nanosheets were dissociated. The effects of the intermolecular forces between the substrate surface and the nanosheets were examined. The work is currently published in the journal, Advanced Materials Interfaces.

The research team showed the orthogonal sectioning of titania nanosheets into substantial rectangular-shaped fragments upon physical adsorption on nonflat surface. During deposition, nanosheets stayed flat on the solvent surface to facilitate the adsorption on the bumpy surface. Thus, the nanosheets were subjected to tensile stress along the lateral direction since the nanosheet area was relatively smaller than the actual area. This tensile stress was induced by the intermolecular forces acting between the substrate surface and the nanosheet upon adsorption process.

The high 2D anisotropy induced significantly large intermolecular forces sufficient to cleave all the chemical bonds across lattice planes orthogonal to the lateral direction of the nanosheets. This resulted in the scission of the nanosheets and the formation of crevices within the nanosheets. The resultant crevices indicated scission along crystallographic axis due to smaller total dissociation energy. Scission was achieved for all the large nanosheets on the substrate surface. Furthermore, the aspect ratio of the nanosheets was among the factors influencing the scission through integrated intermolecular forces.

In summary, scission of 2D nanosheets along the crystallographic axis using spin-coating method was successfully developed. Although the magnitude of the intermolecular forces was much smaller than the dissociation energy, the total intermolecular force exceeded the total dissociation energy across the vertical lattice plane when 2D nanosheet was adsorbed on the substrate. The titania nanosheet sectioning principle demonstrated here is also applicable to 2D materials with other structures and compositions as well as 1D anisotropic materials. In a statement to Advances in Engineering, Dr. Nobuyuki Sakai stated that their findings hopefully will provide a powerful large-scale processing technique for nanomaterials.