61st Annual Report on Research 2016

Two-dimensional (2D) materials have drawn a lot of attention in scientific research because of their excellent chemical and physical properties. Despite the great amount of efforts have been made, however, this research field remains limited by the challenges on the large-scale production and the challenges on developing new materials with structural versatility. In the past year, we have been making progress on tackling the challenges of solubility for potential 2D polymers using a molecular engineering approach. We have made significant progress in exploring the synergy of interfacing organic molecule with inorganic 2D materials for the future development of 2D polymer using inorganic templates.

1. Investigation of cleavable solubilizing groups

We hypothesized that the need of a solubilizing group will be crucial to facilitate the synthesis of 2D polymers. It is also important if the solubilizing group does not interrupt with the 2D sheet once the polymer is formed. In this context, is would be ideal if a group can assist the solubilizing process but can also be cleaved on demand. Tert-Butyloxycarbonyl (Boc) group represents a good candidate for this purpose. A series of solution processable conjugated random copolymers with latend H-bonds were synthesized by Stille coupling reactions. Boc was introduced because it could be easily removed by heating to give H-bonding donor and acceptor in polymer backbone. In addition, Polyisobutylene (PIB) side chain was introduced to give sufficient solubility without compromising electronic performance. By post fabrication thermal treatment, the t-Boc group could be removed efficiently to generate a compact 2D H-bonding network in the solid state, leading to a solvent resistant semiconducting thin film. This unique property renders these polymers novel solvent-resistant materials applicable in harsh conditions. OFET devices demonstrate that the annealed films maintained the charge carrier mobility even after being immersed in different solvents. In addition, the latent H-bonding protected materials by t-Boc also have potential in making very thick film by solution processing and multilayer device.

2. Interfacing molecular species with 2D inorganic sheets

2D inorganic materials possess well-defined morphology that is suitable for templated synthesis of 2D organic polymers. The interfacing and synergy between 2D inorganic materials and organic molecules, however, have not been investigated extensively so far. We selected MoS₂ 2D sheet as the model material to study the interaction with conjugated organic molecules. In collaboration with Dr Sarbajit Banerjee, we developed a stepwise
synthetic approach comprising vapor transport, reduction, and
topochemical sulfidation for creating arrays of 2D MoS₂
nanosheets directly integrated onto carbon fiber paper (CFP)
substrates. The
obtained materials characterized by a high density of exposed
edge sites exhibit promising electrocatalytic performance, including an overpotential (η₁₀) of 245 mV at 10 mA/cm², a Tafel slope of 81 mV/dec, and a turnover frequency (TOF) of 1.28 H₂/s per active site at −0.2 V vs RHE in a 0.5 M acidic solution. The electrocatalytic properties of the MoS₂ nanosheets are observed to be substantially enhanced by interfacing with solution-deposited buckminsterfullerene nanoclusters (nC₆₀). A coverage of 2% of nC₆₀ yields a hybrid electrocatalyst exhibiting an η₁₀ value of 172 mV, a Tafel slope of 60 mV/dec, and a TOF value of 2.33 H₂/s per active site at −0.2 V vs RHE. The enhancement of electrocatalytic activity is found to derive from interfacial charge transfer at nC₆₀/MoS₂ p−n heterojunctions. The high conductivity of the interfacial layer formed as a result of charge transfer from the surface bound nC₆₀ molecules to MoS₂ is thought to substantially mitigate the limitations imposed by the poor basal plane conductivity of undoped 2D MoS₂ sheets. The hybrid catalysts illustrate an important design principle involving the use of structured interfaces to enhance the catalytic activity of low-dimensional materials. Moreover, this work demonstrated the synergy and close electronic coupling between 2D MoS2 sheets and molecular species, opening the opportunity to template the assembly and reaction of organic monomer on 2D materials for the potential bottom-up synthesis of 2D polymers.