Geometry-Controllable Templates from PAA

PAA formed by anodization has been widely studied in more than 100 years.^44-50 Porous alumina membranes are used for the fabrication of composites in nanometer scale because of their relatively regular structure with narrow size distribution of pore diameters and interpore spacings.⁴⁴ The pore structure is a self-ordered hexagonal array of cells with cylindrical pores of variable sizes with diameter of 25 nm to 420 nm with depths exceeding 100 mm depending on the anodizing conditions used.⁵¹

Essentially, the structure is a result of several coupled phenomena. One mechanism is a nonuniform electric field and, hence, current that arises in the porous aluminum structure as a result of topological variations. The second mechanism is either field-enhanced dissolution or increased local temperature that enhances dissolution of the bottom oxide barrier layer.⁴⁴ These coupled phenomena preferentially remove the oxide at the bottom of the pores while leaving the pore walls intact. In another hand,

―Nature‖ describes a model of PAA growth based on simple concepts of volume and charge conservation, coupled with experimentally validated descriptions of interfacial reactions and transport processes recently.⁵² The resulting structure is an ordered hexagonal array of cells with cylindrical pores with cell walls composed of alumina.

Nanoporous anodic aluminium oxide with self-organized hexagonal arrays of uniform parallel nanopores has been used for various applications in the fields of sensing, storage, separation, and the synthesis of one-dimensional nanostructures.^53-55 Self-ordered PAAs have been obtained by mild anodization (MA) and hard anodization (HA). Both of their advantages are utilized in our experiments.

In ordinary two-step mild anodization (MA) process, the self-ordered columns of alumina nanopores can be obtained within three well-known growth regimes:

(1) Sulphuric acid (H2SO4) at 25 V for Dint (interpore distance) = 63 nm^51,56 anodizing time (16 h) was required to obtain highly ordered PAA films.⁵⁶ Masuda et al.

reported self-organized pore growth, leading to a densely packed hexagonal pore structure for certain sets of parameters.⁵⁷ The self-organized arrangement of

neighboring pores in hexagonal arrays can be explained by any repulsive interaction between the pores.

In general, the fabrication of self-ordered AlO pore arrays, under conventional so-called ‗mild anodization‘ (MA) conditions, requires several days of processing time and the self-ordering phenomenon occurs only in narrow process windows, known as ‗self-ordering regimes‘.^51,56-60 Owing to the slow oxide growth rates (for example: 2–6 μm h⁻¹), MA processes based on Masuda‘s approach have not been used in industrial processes so far. Hence hard anodization (HA) of aluminum, a faster process that was invented in the early 1960s is an attractive alternative.^62-63 HA is carried out at relatively low temperatures and high current densities, and has routinely been used in the aluminum industry to produce anodic films of high technical quality at an efficient rate of production (typically 50–100 μm h⁻¹).

Above all, from a practical point of view, the HA process has many advantages over conventional MA. The major findings on the HA process are as follows. (1) The current density (that is, the electric field strength E at the pore bottom) is an important parameter governing the self-organization of oxide nanopores in a given anodization potential. (2) A new self-ordering regime is established over a broad range of Dint = 200–300 nm in C2H2O4 and Dint = 320 nm in H3PO4-H2O-C2H5OH.⁶⁴ (3) The ratio δ of the Dint to the anodization potential is lower (δHA = 2.0 nm V⁻¹ for HA, and δMA

= 2.5 nm V⁻¹ for MA). (4) The porosity P is lower (PHA ~3%, PMA ~10%). (5) The growth rate of the porous oxide film is 30 times larger (>50 μm h−1) than for MA.

(6) .Ideally ordered alumina membranes with a high aspect ratio (>1,000) of uniform nanopores can be fabricated by HA of pre-patterned aluminum. (7) Pulse anodizations of aluminum were conducted under potentiostatic conditions by using sulfuric acid or oxalic acid. Pulses consisting of a low-potential pulse followed by a high-potential pulse were applied to achieve alternating MA and HA conditions. A combination of

HA and MA allows modulation of the pore diameter over extremely high aspect ratios.^61-65

These results imply that pore initiation and the steady growth of alumina cells are strongly influenced by the high current density. In order to explain the mechanism behind pore formation phenomenon of self-organization in HA mechanism, the situation during steady state pore growth has to be considered. Pores grow perpendicular to the surface with the equilibrium of field-enhanced oxide dissolution at the oxide/electrolyte interface and oxide growth at the metal/oxide interface.⁶⁶ While the latter is due to the migration of oxygen containing ions (O^2-/OH^-) from the electrolyte through the oxide layer at the pore bottom, Al³⁺ ions which simultaneously drift through the oxide layer are ejected into the solution at the oxide/electrolyte interface. The fact that Al³⁺ ions are lost to the electrolyte has been shown to be a prerequisite for porous oxide growth, whereas Al³⁺ ions which reach the oxide/electrolyte interface contribute to oxide formation in the case of barrier oxide growth. A possible origin of forces between neighboring pores is therefore the mechanical stress which is associated with the expansion during oxide formation at the metal/oxide interface. During the initial stages of film growth, the penetration paths that develop (which are the precursors of the regular pores) are more densely distributed due to the high anodizing current densities. Since the oxidation takes place at the entire pore bottom simultaneously, the material can only expand in the vertical direction, so that the existing pore walls are pushed upwards. Thereafter, steady film growth is attained with the development of the major pores and the repulsive interaction between the alumina cells. The repulsive interaction force, associated with expansion during film formation at the aluminium/oxide interface, increases with electric field. The strong repulsive or expansion force (high field) under high current density limited the transverse growth of alumina cells and forced them to form

close-packed hexagonal arrays. Thus producing highly ordered PAA films is over a large area.⁶⁷

The anodic porous alumina, which is formed by Al anodization in acidic solution, is a typical self-ordered material. Under appropriate anodization conditions, long-range-ordered anodic porous alumina with an ideally pore size can be obtained.

The shape of the holes in the anodic porous alumina can be controlled by a process composed of a series of anodization and subsequent etching treatments (H₃PO₄). By using the anodic porous alumina with shape-controlled holes as a mold for the replication, the preparation of high aspect-ratio structures of polymer could be achieved.⁶⁸

3.2 Materials and Methods

Chemicals and Materials. Phosphoric acid (H3PO4, 86%), anhydrous ethanol (99.5%), and dihydrate oxalic acid (C2H2O4·2H2O, 99.8%) were purchased from J. T.

Baker (Phillipsburg, NJ, USA). Polyurethteneacrylate (soft PUA, 99%) was purchased from Sigma-Aldrich (St. Louis, MI, USA). Chromium trioxide (CrO3, 99%) and perchloric acid (HClO₄, 70%) were purchased from Showa (Tokyo, Japan).

Aluminium foil (99.999%) and 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane (C₈H₄Cl₃F₁₃Si, 97%) were purchased from Alfa Aesar. Ultrapure water used in all experiments was purified with a Milli-Q apparatus (Millipore, Billerica, MA, USA) to a resistivity of 18.2 MΩ cm. All chemicals were used without further purification.

Anodization Process. Aluminum foils were degreased in acetone, washed in de-ionized water, and put into a tailor-made holder with an opening of 3 cm². Before anodizing, the aluminum was treated with annealing at 450^oC for 12 hr and electropolished at a constant voltage in a 1:4 volume mixture of perchloric acid and ethanol under room temperature. The C2H2O4–C2H5OH–H2O electrolytes, and

H₃PO₄–C2H₅OH–H₂O electrolytes are designated. For the pre-step HA, the concentrations of phosphoric acid in the electrolytes were in the range of 0.25 M. The temperatures of the electrolytes were kept at −10°C by a powerful low-constant-temperature bath, and samples were anodized at target voltages 195 V for 10 min. After the selective removal of alumina in a mixture solution of CrO₃ and H3PO4, the first HA was carried out under the same anodization conditions. During the first anodization step, the shape of the holes was precisely controlled by changing the times of anodization and etching in phosphoric acid at 53°C. For the second step HA, the concentrations of oxalic acid in the electrolytes were in the range of 0.1M, the temperatures of the electrolytes were kept at -10 °C, samples were anodized at target voltages 195 V for 10 s, the PAA template with pore diameter of 380 nm and length of 1.3 μm was obtained. A Cu foil was used as the support for the working electrode. The substrate was positioned on top of the support while Cu tapes were used to connect the Cu support to the Al film upon the sample. The design advances the uniform current distribution from Cu support to underlying Al, leading to a homogeneous anodization. Two acrylic caps were used to fasten the sample and the Cu support inside within O-ring.

Fabrication of Hairy Nanostructures. The taper shaped gecko-like structure via self-ordered porous alumina hard templates serving as shape-defining molds is well-established. After preparing the tapered PAA master with a specific feature size, we fabricated tapered nanohairs by replicating the master with soft UV-curable polyurethane acrylate (PUA). The master was treated with a fluorinated SAM solution.

The treated master mold was annealed at 120 ⁰C for 20 min. Drops of soft PUA prepolymers were dispensed onto the master mold and UV-transparent able silica was slightly pressed against the liquid drop as a supporting backplane. After preparing a polymer replica by UV exposure for 1hr through the tranparant back side (dose = 15

mJ/cm²) and mold removal, the PUA replica was additionally exposed to UV for 10 h for complete curing.

3.3 Results and Discussions