Inorganic materials including ceramics or metals are the main components of most functional devices used for energy conversion, catalysis, and electronic displays. Due to their crystalline nature and strong covalent ionic bonds, these materials are stiff and strong and require high temperatures or pressures to be processed. In contrast, organic materials like polymers and biomolecules are highly flexible and can exhibit liquid-like properties and self-assemble under ambient conditions. Synthesizing inorganic materials with polymer-like properties would thus facilitate the fabrication of multifunctional devices for a variety of practical applications, especially in the optical and energy fields.

As described in ACS Materials Letters (doi:10.1021/acsmaterialslett.0c00149), S. Zhang and X. Wang from Tsinghua University, China, propose to realize such materials using sub-1-nm inorganic materials. Such nanomaterials have a high-aspect ratio with one dimension under 1 nm. With a higher number of atoms directly at the surface, as compared to bulk atoms, inorganic nanowires, nanocoils, or nanohelices exhibit excellent optical and catalytic properties.

Similar to polymer chains, sub-1-nm materials have low crystallinity and low internal cohesive energy, of about tens of kJ.mol^–1. This is orders of magnitude lower than the energies of covalent and ionic bonds of several hundreds and thousands kJ.mol^–1, respectively. With such a low internal energy, sub-1-nm nano-wires (SNWs) are not only flexible, they can also self-assemble via weak surface interactions such as Van der Waals and hydrogen bonds. Several recent studies have reported bendable and flexible ultra-high-aspect-ratio nanowires made of tungsten oxides, indium sulphide, gadolinium oxyhydroxide (GdOOH), to name a few, that can self-assemble into ordered structures (see Figures a and b).

With polymer-like morphologies and physical properties, SNWs also share similar processing methods. For example, when suspended in solvents, they form viscous fluids that can flow under shear. By applying wet methods like electrospinning or wet spinning, non-woven meshes can be obtained. Meshes fabricated using GdOOH SNWs retain the high modulus and tensile strength of each individual inorganic fiber, of 10.3 GPa and 712.5 MPa, respectively, while being flexible macroscopically (see Figures c and d).

The processability and flexibility of sub-1-nm inorganic materials make them particularly interesting for applications in optics. For example, the high anisotropy of SNWs and their inorganic character can be used to create age- and moisture-resistant films that are birefringent in the visible spectrum and polarized UV light. Some SNWs like those obtained with GdOOH also have chiral properties and self-assemble into macroscopic hel-ices through evaporation-induced self-assembly. When the GdOOH SNWs are combined with a fluorescent dye, chiral fluorescence can be recorded from the helicoidal structures (see Figure e). This may open unexplored fields of research.

Combining functional performance from the inorganic components with the structural properties of polymers is expected to lead to many applications. For example, “flexible solar cells have been limited in their applications due to their poor bending capacity. The approaches described by Zhang et al. have thus exciting implications for tailored support structures that improve the flexibility and durability of solar cells,” say Michel Nasilowski and Dane deQuilettes from the GridEdge Solar research program at the Massachusetts Institute of Technology.

Sub-1-nm inorganic materials also have the potential to achieve multifunctionality and performance with recyclability and less chemical waste. “The synthesis procedures of such nanomaterials are very simple … while using ordinary equipment and nontoxic solvents,” Wang says.

Originally published in the July 2020 issue of MRS Bulletin.