Recent Progress on Monoelemental Nanomaterials with Unconventional Crystal Phases

As an important parameter of crystalline materials, the crystal phase describes the periodic atomic arrangement in their structures. For some monoelemental materials, e.g., carbon and phosphorus, they can exist in more than one crystal phase. The different crystal phases of monoelemental materials result in different physicochemical properties and functions. Therefore, engineering the crystal phase of monoelemental materials gives an effective strategy to modulate their properties and functions. Conventionally, the crystal phase of monoelemental materials can be altered under some harsh conditions, e.g., high temperature and high pressure. Recently, with the rapid development of nanotechnology, various monoelemental materials with unconventional crystal phases have been well developed on the nanoscale. For example, our group has successfully achieved the synthesis of Au nanomaterials with unconventional hexagonal close-packed (hcp) 2H and 4H phases by wet chemical methods, which exhibit distinct optical properties as well as outstanding electrocatalytic performance when compared to those with a thermodynamically stable face centered cubic (fcc) phase.In this Account, we give a comprehensive overview of the recent development of monoelemental nanomaterials with unconventional crystal phases and their crystal-phase-dependent properties and applications. We first introduce the typical strategies for the synthesis of monoelemental nanomaterials with unconventional crystal phases. By using a wet-chemical reduction method, template-assisted method, and thermal annealing method, monoelemental nanomaterials with unconventional crystal phases can be directly prepared. Besides, unconventional-phase monoelemental nanomaterials can also be obtained via the phase transformation from materials with conventional crystal phases under specific conditions. In addition, some other methods have also been reported for preparing monoelemental nanomaterials with unconventional crystal phases, such as controlled crystallization of amorphous structure, the chemical vapor transport (CVT) method, electrodeposition, galvanic replacement, sputter-deposition, and so on. Subsequently, we summarize the unique structural stability and magnetic, electronic, optical, and other properties of the obtained monoelemental nanomaterials with unconventional crystal phases. We also highlight their promising applications in catalysis and batteries. Finally, we present our personal perspectives on the challenges and future opportunities in this important research field.