Imogolite类纳米管直径单分散性密度泛函理论研究

Diameter monodispersity of imogolite-like nanotube:a density functional theory study

采用密度泛函理论方法研究了三种imogolite类(未取代、NH_2取代和F取代)纳米管的直径单分散性及表面电荷的分布情况,并从键长方面定性地解释了直径单分散性的原因.我们给出了IMO,IMO_NH_2和IMO_F的应变能曲线,结果表明三种纳米管结构的最稳定管径值按照IMO<IMO_NH_2<IMO_F的顺序递增,而imogolite类纳米管直径单分散性是由于管径的增大导致内部Si—O,Al—O键与外部Al—OH键键长变化趋势相反造成的,总之是内部Si—O,Al—O键和外部Al—OH键相互作用的结果.此外,对三种稳定的纳米管结构做了Mulliken布局分析,并总结了纳米管直径变化对表面电荷的影响.结果表明正电荷主要积聚在外表面,而内表面则感应出负电荷,同时随着纳米管直径的增大表面电荷逐渐增加,揭示了表面电荷与管径大小的关系.研究表明,可以通过改变imogolite内表面不同的官能化取代来控制纳米管直径,进而调节表面电荷的分布情况,这在imogolite类材料的分子设计及应用方面有着重要意义.

The diameter monodispersity and the surface charge distribution of three imogolite-like nanotubes （not substituted （IMO）, substituted by NH2 （IMO-NH2）, substituted by F （IMO-F）） are investigated using self-consistent periodic density functional theory, and the phenomenon of the monodispersity is explained qualitatively in terms of bond length. We assume that the axial length of the nanotube is constant and confirm it; the energetic minimum axial lengths of the three nanotubes increase in the sequence IMO NH2〈 IMO 〈 IMO F, and are respectively 8.61, 8.62 and 8.66 A. Then the energies for different nanotubes and lamellar structures are calculated. A series of strain energy curves of IMO, IMO NH2 and IMO F are plotted based on calculations, and the results show that the energetic minimum diameters of these three nanotubes increase in the sequence of IMO 〈 IMO NH2〈 IMO F, and are respectively N = 9, 10 and 11. In order to explain the diameter monodispersity, we have calculated the bond lengths of Si O, Al-- O and Al- OH three nanotubes and plotted the curves of length against diameter. Results show that the monodispersity can be attributed to the interaction between the energy increase resulting from the stretching of the Si--O, Al--O bonds in the inner wall, and the energy decreases caused by the shortening of the Al OH bond in the outer wall. In a word, with the increase of tube diameter, the Si--O and Al--O bonds increase while the Al--OH bond decreases monotonically. Additionally, we have also calculated the Mulliken charge distributions of the three nanotubes with different diameter and analysed their surface charges. On this basis, we summarize the effect of diameter on surface charge. Results show that the main positive charges are accumulating on the outer surface while the negative charges are located on the inner region, and the outer surface charge increases gradually with the increase of the diameter of the nanotubes. The study indicates that the internal surface functional group has an effect on the axial length, diameter and surface charge of the imogolite- like nanotubes. We can control the nanotube diameter and surface charge distribution by changing different flmctional substitutes in the inner surface; it is significant in the molecular design and application of imogolite-like materials.