Understanding topology as a chemist
So far, the discussion of topological materials and topological electronic properties is still largely confined within condensed matter physicists. In fact, the concept of topology can be understood more intuitively from the perspective of a chemist.
Catalyst using topological materials
Despite tremendous progress of topological material search and studies, it is still unknown how these topologically protected electronic states interact with other particles/molecules in the context of electrochemical reactions and how they respond to ambient/coherent light in the context of photosynthesis and photochemistry. In topological insulators and metals, the existence of topologically protected electronic states and quantum Berry phase may give rise to entirely new physical chemistry properties, which are not possible in their conventional counterparts. One of such potentials is to use topological materials as catalysts for chemical reactions.
The Earth’s atmosphere provides a universal feedstock of water, carbon dioxide and nitrogen, which can potentially be converted into clean and sustainable fuels including hydrogen, oxygenates and ammonia via the electrochemical processes. Therefore, identifying efficient and low-cost electrocatalysts plays a central role in clean energy conversion. It has been widely recognized that Pt is the best-performing catalyst for major electrochemical processes such as hydrogen evolution reaction (HER). However, the scarcity and high cost of Pt could limit its widespread technological use. This has sparked a search for Earth-abundant catalysts that potentially could replace Pt. Current strategies to improve the activity (or reaction rate) of an electrocatalyst system relied on increasing the number of active sites on a given electrode or increasing the intrinsic activity of each active site. Based on these strategies, many inorganic materials such as the MoS2 family semiconductors, 3d transition metals Ni, Co, and their phosphides/sulfides (CoP, CoS2) have been tested.
Our group will explore new design principles by utilizing topological electronic states to spur catalytic activity. Surprisingly, although the original research on topological phases of matter was motivated by fundamental physics and abstract math, we realized that many of the unique electronic properties of topological materials are highly desirable for an ideal electrocatalyst. First, topological materials feature linear (photon-like) band crossings in their band structures. As a result, they are highly conductive with an exceptional electron mobility. Moreover, the high electron mobilities reduce carrier recombination, leading to higher efficiency. Second, topological materials host protected, metallic surface states, such as the Fermi arc surface states in topological Weyl metals and the Dirac surface states in topological insulators. The existence of metallic surface states near Fermi level can significantly enhance adsorption, desorption, and all kinetic processes, facilitating the desired reaction. In contrast to conventional surface states due to band bending or dangling bonds, the topological surface states connect across the bulk band gap. Therefore, they cannot be removed/passivated and are hence immune to contamination. These properties suggest that topological materials can be excellent catalysts. Indeed, a recent experiment has shown that the Weyl metal NbP (a member of the TaAs family that is relatively earth abundant), without deliberate optimizations, is already highly efficient catalysts for HER. Considering the large number of known topological materials, identifying ideal catalysts and understanding the underlying mechanisms represent an exciting, emerging field.
Reading:
Weyl Semimetals as Hydrogen Evolution Catalysts , Rajamathi et. al., Adv. Mater. 29, 1606202 (2017)