Bottom-up synthesis of high entropy hydroxide nanosheets and their potential applications
LI F. 1, ABE H. 1
1 Joining and Welding Research Institute Osaka University, Osaka, Japan
Doping is an effective method to develop novel materials with improved properties. The pioneering work of Yeh and Cantor independently on high-entropy alloys (HEAs), which describe a class of single-phase materials containing five or more elements in relatively high concentrations (5-35 at.%), change the way to explore novel materials from around the corners or edges to the centers of phase diagrams. As the number of elements in an alloy increase, the entropic contribution to the total free energy would overcome the enthalpic contribution and stabilize the multiple elements in a single-phase solid solution. The concept of high entropy expands to ceramics field since 2015. The concept of high-entropy design opens a door to develop novel ceramics with vast compositional space. Numbers of high-entropy ceramics (HECs), including high-entropy oxides, high-entropy hydroxides (HEHs), high-entropy carbides, high-entropy borides, and high-entropy silicides, have been successfully prepared. The multi-element feature of HECs leads to several advantages compared to their low-entropy counterparts, which can be categorized into four core effects, namely high-entropy effect, sluggish diffusion effect, severe lattice distortion effect, and cocktail effect.
Low-dimensional high-entropy materials (e.g., nanosheets and nanoparticles) have great potential for wide applications. The combination of unique surface features of nanosheets and four core effects of high-entropy materials brings great possibilities to discover novel multifunctional materials. However, it is still challenging to synthesize high-entropy nanosheets through a bottom-up soft chemistry method, due to the difficulty of mixing and assembling multiple elements in 2-dimensional (2D) layers. Here, we report a simple polyol process for the synthesis of a series of 2D layered high-entropy transition metal (Cr, Mn, Fe, Co, Ni, Zn) hydroxides (HEHs), involving the hydrolysis and inorganic polymerization of metal-containing species in ethylene glycol media. The formation of metal-polyol complexes facilitate mixing of multiple elements at mild conditions. Hydrolysis and reduction of metal cations, which can be controlled by solvothermal temperature, time, and solution pH, are two competitive reactions during polyol process. By optimizing the synthesis conditions and simply changing combination of metal cations, the HEH nanosheets can be successfully obtained after solvothermal treatment at ~200 °C. The surface of these HEH nanosheets are highly defective, demonstrating promising electrochemical catalytic activity for oxygen evolution reaction. These HEH nanosheets can adsorb anionic dyes from wastewater, possessing adsorption kinetics that are two orders of magnitude higher than that of commercial activated carbon. Our observations reveal that utilizing configurational entropy to modulate the electronic structures and defects would be an effective approach to obtain novel materials with charming applications. Upon post annealing at low temperatures (<300 °C) in air, the polyol process can also be used to prepare high-entropy oxide nanoparticles (~ 10 nm) with various crystal structures (e.g., spinel, pyrochlore and fluorite).