Method to scale-up fabrication of 2D materials

Could a new exfoliation method offer a practical route to large-scale production of nanosheets?  

Ever since graphene first made headlines in 2004, the search for alternative 2D materials has turned into a global research effort. Thanks to their atomically-thin dimensions, 2D materials can offer unique combinations of interesting properties For example, graphene is optically transparent, but shows remarkable tensile strength and electrical conductivity. Another 2D material that’s under intense study is hexagonal boron nitride (hBN), often erroneously called ‘white graphene’.

Thanks to its ultra-wide band gap, hBN is an electrical insulator. It is also thermally conductive and remarkably chemically stable. In its bulk powder form, hBN has long been used in everything from cosmetics to industrial lubrication, but as a 2D material, it shows particular promise for use in enhancing the efficiency of photocatalysts for water treatment. Current methods to exfoliate hBN nanosheets from the bulk material often result in low production yields, however. So, a group of Chinese researchers decided to tackle that, and they have just published their results in Materials Today [DOI: 10.1016/j.mattod.2018.10.039]

One of the challenges of exfoliating hBN is the partial ionic bonding that exists between individual nanosheets of the material. The team, led by Prof Ching-Ping Wong, hypothesised that adding lithium ions (Li⁺) under high pressure conditions would lead to an expansion of the structure. The idea was that the cations would insert themselves in between the hBN layers, reducing the amount of energy needed to break the ionic bonds. And by choosing an appropriate solvent, they could reduce the exfoliation energy even further, making it much easier to then exfoliate individual, high-quality nanosheets.

They analysed five different solvents, each time carrying out a hydrothermal exfoliation of hBN at 180 °C and a pressure of 2 x10⁶ Pa. The researchers found that isopropyl alcohol (IPA) provided the highest yield, and when combined with strong stirring, they could reach yields of 54%. To test their lithium intercalation theory, they then added lithium chloride – a good source of Li⁺ ions – to the solvent. The addition improved the exfoliation yield to 55% and resulted in a significantly higher concentration of hBN nanosheets. In addition, the lateral size of the exfoliated nanosheets was higher with Li⁺ intercalation than without.

In order to investigate the photocatalytic properties of the exfoliated hBN nanosheets, they were added to titanium dioxide (TiO₂) and tested against the dye, methyl orange. Wong and his colleagues found that the photocatalytic activity of TiO₂ was greatly enhanced by the addition of hBN, suggesting that it acts to improve the catalyst’s light absorption behaviour.

Perhaps most significantly, the team applied their hydrothermal exfoliation method to a range of 2D materials, including tungsten disulfide (WS₂), graphite, molybdenum disulfide (MoS₂), and vanadium oxide (VO₂). With the appropriate solvent, all could be transformed into their corresponding nanosheets. This implies that their simple method could be a universal approach for the exfoliation of 2D materials.

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Ning Wang, Guang Yang, Haixu Wang, Changzeng Yan, Rong Sun, Ching-Ping Wong. “A universal method for large-yield and high-concentration exfoliation of two-dimensional hexagonal boron nitride nanosheets” Materials Today XX (2018) XX-XX. DOI: 10.1016/j.mattod.2018.10.039