Ligand-free synthesis of noble metal nanocatalysts for electrocatalysis

Noble metal nanocatalysts have shown excellent activities for electrocatalysis. The commonly used method to synthesize these nanocatalysts is the colloidal synthesis, during which ligands are necessary. The problem in the colloidal synthesis is that post treatments (such as thermal annealing) are usually needed to remove the residual ligands to clean the surface of nanocatalysts. These post treatments could alter the surface structure of the prepared NPs and deactivate the catalytic performance. Therefore, it is interesting to directly prepare the catalysts with ligand-free properties. In fact, ligand-free synthesis of noble metal nanocatalysts has attracted much attention during the past decade. In this review we comprehensively overview the ligand-free synthesis of noble metal nanocatalysts and their electrocatalytic applications. Three synthetic methods for ligand-free synthesis (stabilization by small molecule, stabilization by modified support and constructing self-supported NPs) are classified and typical examples are given. The advantages and disadvantages of these synthetic methods are compared. Different approaches to characterize the ligand-free property of synthesized noble metal nanocatalysts are introduced and compared. Because of the ligand-free property of the synthesized nanocatalysts, they often exhibit excellent electrocatalytic activities. Therefore, the applications of as-synthesized ligand-free nanocatalysts for energy-related electrochemical reactions are summarized: liquid fuel oxidation reaction (LFOR), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO2RR), nitrogen reduction reaction (NRR). Ligand-free synthetic process is often simple and efficient, which largely reduces the cost of catalyst synthesis, and can be more suitable for industrial production. However, there are still some problems to be solved in the future. Therefore, we point out the current underlining problems and give the perspectives in this emerging field.