The discovery of new materials has played an enabling role in all aspects of technical progress, ushering in new energy, water, health, and information technologies. Solid state transformative synthesis, including topochemical reactions at room or elevated temperatures where one phase is converted to another while maintaining a defined crystallographic relationship between the precursor and product phases, offers significant opportunities to produce novel materials, including metastable and hierarchical structures unavailable to synthetic processes that start from a vapor or liquid phase. In contrast to traditional inorganic synthesis approaches in which the constituent elements are brought together to facilitate formation of the stable equilibrium structure, transformative synthesis starts with a stable material exhibiting a similar composition or similar structural elements to the desired product. This starting phase is transformed through selective atomic removal, insertion, or exchange reactions at conditions that kinetically forbid global structural changes or material decomposition to yield a new non-equilibrium material. The moderate temperature and far-from-equilibrium nature of these transformational processes is central to avoiding equilibrium phases that form at higher temperatures.<br/>My first venture into this field was synthesis of carbide-derived carbon with tunable pore size by hydrothermal extraction of Si from SiC and by chlorination of many other carbides. Non-catalytic synthesis of graphene, graphite and nanotubes by extracting Si from SiC in vacuum followed. Layered graphene-like and carbon-sulfur structures were produced by electrochemical etching of layered ceramics known as MAX phases. A diverse family of two-dimensional (2D) transition metal carbides, nitrides and carbonitrides (MXenes) was discovered in 2011. The MAX to MXene transformation through selective extraction of A-site elements is paradigm shifting since it was one of the first examples wherein a 3D solid, with quite strong primary bonds, was converted into a 2D counterpart, and this was done for dozens of 2D materials. However, many more have been predicted and an infinite number of solid-solution MXenes can possibly be made. Transformative synthesis may open a new era of synthetic control over non-equilibrium nanoscale materials for energy related and many other applications.

Transformative synthesis of inorganic materials: Lessons from three decades of research

ACS Fall 2021

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