超薄Ni–Fe氢氧化物纳米片作为催化剂的磁性和电子性能的条件:DFT+U研究（英文）

Conditions for magnetic and electronic properties of ultrathin Ni–Fe hydroxide nanosheets as catalysts:a DFT+U study

利用密度泛函理论(DFT)+U方法,本文研究了单层Ni–Fe氢氧化物纳米片中阳离子排列方式和表面补偿阴离子对其磁结构和催化性能的影响.Fe原子偏聚引起的无序结构会在纳米片中产生不同自旋排列的磁畤,而表面补偿阴离子会影响Fe原子的局部磁矩和价态.这两种实验条件不会从根本上改变Ni–Fe氢氧化物纳米片中磁性来源,即磁性金属原子间的超交换作用本质,但可在纳米片中形成特性各异的磁性超晶格.此外,氧还原/析出反应中间态自由能变化的计算证实最稳定磁结构的Ni–Fe氢氧化物纳米片具有优异的催化性能,表现出磁催化剂的特性.其原因是反应中Fe^(2+)–Fe^(3+)间价态的循环转变,决定了O–H键断裂和OH基释放的难易程度,从而控制了反应基元限制步的速率.我们可以通过Fe离子价态变化将纳米片的磁性和催化活性联系起来,该认识将有助于打开氢氧化物/LDHs为基的磁性催化剂设计之路.

The effects of cation ordering and surface compensating anions on the magnetic structure and catalytic properties of unilamellar Ni-Fe hydroxide nanosheets are studied by using the density functional theory （DFT） plus U method. Fe-segregation in the nanosheets yields magnetic domains with different spin alignments, while the surface compensating anions affect the local moments and valence states of the Fe atoms. The two conditions do not radically change the super-exchange nature of interactions between the paramagnetic metal centers, hut facilitate the formation of various magnetic superlattices in the nanosheets. The calculated free energy change of the intermediates shows that the most stable magnetic structure of Ni-Fe hydrox- ide nanosheets exhibits superior catalytic activity towards oxygen reduction/evolution reactions, which is indicative of magnetic catalyst. This is due to the cycle transition between Fe2＋ and Fe~~ ions in the reactions, which determines the sequence of cleavage of the O-H bond and the release of the OH group, controlling the rate-limiting steps of the reaction. The relationship of magnetism and catalytic activity of Ni-Fe hydroxide nanosheets is established by the valence state change of the Fe ions, which will be helpful to open the way for the design of hydroxide/layered double hydroxides （LDHs）-based magnetic catalysts.