Phase and morphology evolution of well-dispersed nickel phosphide nanocrystals and the abnormal ferromagnetic origin by experimental and theoretical studies

Thermal decomposition strategy has been widely used to fabricate well-dispersed size/shape-controlled metal phosphides, which have versatile structural and electronic characteristics. However, the underlying formation mechanism is still unclear. Herein, taking nickel phosphides as the example, the phase/morphology regulatory mechanism was studied in detail, and the intrinsic origin of eccentric ferromagnetism was investigated. The main findings are as follows: 1) The interdependent synthetic levers can manipulate the phase conversion from tetragonal Ni12P5 to hexagonal Ni2P, with the morphology transformation from small solid to big hollow structure.; 2) The hollow structure is caused by Nanoscaled Kirkendall effect, while the accompanying growing particle size is attributed to Lamer mechanism and Ostwald ripening mechanism.; 3) Formation studies of Ni2P shows that the phase evolution undergo the process of FCC Ni → amorphous Ni–P → tetragonal Ni12P5 → hexagonal Ni2P, during which the morphology converts from small solid to large hollow structure. Dissolution-recrystallization process also plays an important role in the phase, size and shape evolution.; 4) The Ni2P nanoparticles show abnormal size-dependent ferromagnetism, in which larger hollow particles show higher saturation magnetization. Annealing treatment experiments and first principles calculations clarify that the intrinsic ferromagnetic origin of Ni2P is caused by P defect. This work deeply emerges the structural evolution and formation mechanism of nickel phosphides, and certifies the positive effect of P vacancy on the magnetic properties, which gives a sight to tail the metal phosphides to expand their applications.