基于多层结构理论的三维纳米多孔阳极氧化铝实时扩孔机理研究

Real-time studying the mechanism of 3D pore-widening of nano-porours anodic alumina with a multilayer structure

摘要:

多孔阳极氧化铝的孔径大小及其均匀性可通过扩孔进行改善。本文以3wt%磷酸作为刻蚀液，在实现PAA孔径扩大的同时，我们详细研究了三维状况下孔径形态的实时演变过程，发现PAA孔加宽在表面和截面两个方向上同时进行。在表面扩孔研究中，磷酸逐层蚀刻PAA内壁多层结构，随着扩孔时间的增加，扩孔速率不同。在截面研究中，当扩孔时间达到某一定值，由于扩孔速率不同，PAA顶部的扩孔过程较底部的扩孔过程相对较早完成，随后顶部扩孔速率逐渐降低，底部扩孔速率逐渐增加，当扩孔完成时，形成均一性良好的理想圆形小孔。同时，我们研究发现，过度增大扩孔时间，PAA基元结构将被破坏，并由于高纵横比形成纳米线，随后被刻蚀溶液腐蚀。这是首次通过实验对PAA扩孔机理在截面方向的演化过程进行研究。本文详细阐明三维PAA的实时扩孔机制，结合多层结构对机理进一步解释，对PAA薄膜的可控制备和扩孔机理的精准研究是很有意义的。

Abstract:

The uniformity of the pore size and the morphology of nano-porous anodic alumina (nano-PAA) can be improved by a pore-widening process. In this paper, the evolution of the pore diameter and the morphology are investigated in details and in real time. Phosphoric acid solution was used to chemically enlarge the pores of a nano-PAA prepared with oxalic acid. The pore-widening is monitored by carrying out simultaneously both top-view and cross-section analyses. From the top-view analysis, the pore diameter results to be gradually enlarged with the increasing etching time. However, such enlarging rate varies with the multilayer structure of the pore wall. From the cross-section analysis, the pores wall of a multilayered amorphous compound result to be etched in a stage-by-stage mode until almost everything is etched away. The pore-widening procedure on the upper portion is completed earlier respect to the lower portion, after some time the pore-widening becomes uniform along the axial direction. If the etching is maintained, hexagonal pore walls are destroyed and split into ultrahigh aspect ratio nanowires due to the etching anisotropy, and finally completely dissolved. This is the first experimental report on the structural evolution details of alumina nano-pores upon etching, combining analyses in the depth direction with stereoscopic 3D imaging. Results clarify the pore-widening mechanism in 3D nano-PAA and can have important practical applications.