Structural evolution of self-ordered alumina tapered nanopores with 100 nm interpore distance

Abstract

We in-detail investigated the profile evolution processes of highly ordered alumina under the cyclic treatment of mild anodizing of aluminum foils in oxalic acid followed by etching in phosphoric acid. With the cyclic times increasing, the profiles of nanopores were gradually evolved into the parabola-like, trumpet-like and conical shape. Although the inserted etching itself nearly had no impact on the growth rate of the nanopores due to the rapid recovering of thinned barrier layer at the initial stage of next anodizing, overmuch etching could bring apparent side effects such as wall-breaking, thinning and taper-removing from the top down. The anodizing and etching kinetics and their synergetic effects in modulating different aspect ratios and open sizes of conical pores were studied systematically. These findings are helpful to tailor high-quality anodic alumina taper-pores with tunable profiles.

Introduction

Ordered porous anodic alumina (PAA) has attracted intensive interest due to their potential as initial templates for studying special physical and chemical effects of different materials of zero-dimensional (0D) nanodot, 1D nanofiber, nanotube, nanochannel, and nanorod arrays [1], [2], [3], [4]. So far, great breakthrough has been made in the fabrication of ordered PAA nanostructures with controllable interpore distances (d_int) in the range of 50–600 nm by mild anodization [5], [6], [7], [8], [9], hard anodization [10], [11], [12], [13], and those combined with pre-patterning techniques [14], [15], [16]. However, these as-prepared nanopores are nearly cylindrical. Very recently, to develop advanced functional nanostructures, renewed interest was focused on the modulation of internal configurations of anodic alumina nanopores at 3D scale [17], [18], [19], [20].

Alumina taper-pore arrays (TPA) are just a type of emerging and valuable 3D nanostructures [21], [22], [23], [24], [25], [26], [27]. They could be utilized as templates for developing different materials of functional nanocone arrays (e.g., broadband anti-reflection for transparent optical polymers [21], [22], Raman signal enhancement for Au [23], hard mold for nickel [24], [25] and tungsten [26]) and as host structures for controlling the orientation of self-organized silica mesochannels [27]. To date, these reported functions are nearly performed on the basis of self-ordered TPAs with d_int ∼ 100 nm, which are achieved by multi-step C₂H₂O₄ mild anodizing and H₃PO₄ etching [21], [22], [23], [24], [25], [26], [27]. However, this method is time-consuming (>8 h) and period-limiting (63 nm for H₂SO₄ at 25 V, 100 nm for C₂H₂O₄ at 40 V, 500 nm for H₃PO₄ at 195 V), which seriously restrict the abilities to modulate the periods of taper-pores and optimize material's functions [5], [6], [7], [8], [9]. To solve these problems, Yanagishita et al. adopted a mold-imprinting technique to pattern Al foils with regular nanopits, exemplified by d_int ∼ 200 nm, 300 nm and 500 nm, and then in situ grew taper-pores via cyclic H₃PO₄ anodizing and H₃PO₄ etching [28], [29]. Besides, Yamauchi et al. smartly utilized electron moiré fringes to solve the difficulty in characterizing the domain sizes and periodicities of self-ordered TPAs [30]. However, it is still very difficult to precisely custom-tailor different profiles of high-quality taper-pores at this stage [31]. Thus, further investigation into the modulation process of the taper-pores and the synergetic effects of anodizing and etching is essential to grasp the precise tailoring ability. Actually, there are no systematical experimental studies on the growth of ordered taper-pores varied with the cyclic times (CT), anodizing and etching time under specific electrolytes, to the best of our knowledge [21], [26], [27], [32].

Here, we in-detail investigated the effects of electrochemical reaction conditions, such as CT, anodizing and etching duration, on the growth and modulation of taper-pores during the cyclic C₂H₂O₄ mild anodizing and H₃PO₄ etching of Al foils. By fixing the same anodizing and etching condition, we monitored the thickness of taper-pores (T_T-TP) and the barrier layer thickness (T_T-BL) varied with the CT, and current densities of anodization with respect to the time from the 1st to 13th cycles. The dependence of open sizes of pores on the CT is also monitored. It was found that the wall texture and aspect ratios of conical pores could be effectively tuned by controlling the combination of anodizing and etching time. These results clarify the roles of the discrete anodizing and etching and their cooperative effects, which can rationalize the profile evolution of taper-pores. These findings are helpful to custom-tailor different profiles of 3D taper-pores under different acidic electrolytes and develop multifunctional nanomaterials.