Abstract
Self-organized porous anodic alumina (PAA) formed by electrochemical anodization have become a fundamental tool to develop various functional nanomaterials. However, it is still a great challenge to break the interpore distance (Dint) limit (500 nm) by using current anodization technologies of mild anodization (MA) and hard anodization (HA). Here, we reported a new anodization mode named “Janus anodization” (JA) to controllably fabricate self-ordered PAA with large Dint at high voltage of 350–400 V. JA naturally occurs as anodizing Al foils in citric acid solution, which possessing both the characteristics of MA and HA. The process can be divided into two stages: I, slow pore nucleation stage similar to MA; II, unequilibrium self-organization process similar to HA. The as-prepared films had the highest modulus (7.0 GPa) and hardness (127.2 GPa) values compared with the alumina obtained by MA and HA. The optical studies showed that the black films have low reflectance (<10 %) in the wavelength range of 250–1500 nm and photoluminescence property. Dint can be tuned between 645–884 nm by controlling citric acid concentration or anodization voltage. JA is a potential technology to efficiently and controllably fabricate microstructured or hybrid micro- and nanostructured materials with novel properties.
Highlights
It is well accepted that Dint is closely related to the anodization voltage
Different from previous studies in pure citric acid anodization, we found that the anodization can be stably performed at high voltage (400 V) in 1.5 M citric acid
We found that the anodization in pure citric acid can be stably performed at ultra-high voltage by removing the reaction heat from both the aluminum surface and electrolyte, which is a new anodization mode named “Janus anodization” and present distinct features as below: (1) mild anodization (MA)-like pore nucleation process but high current density and take much more longer time; (2) hard anodization (HA)-like self-organization process but moderate decline of the current density; (3) the black film is the product of Janus anodization, which demonstrates good mechanically stability, anti-corrosion ability, low reflectance in the wavelength range of 250–1500 nm and PL property; (4) highly ordered nanodents with large Dint between 645 nm to 884 nm can be obtained by adjusting the electrolyte concentration or anodization voltage
Summary
It is well accepted that Dint is closely related to the anodization voltage. For a certain electrolyte system, the higher the anodization voltage, the larger the Dint[22]. Kikuchi et al found that highly ordered porous anodic alumina with Dint between 530–670 nm could be successfully fabricated under the anodizing voltage of 210–270 V in edtidronic acid, which was a new type electrolyte possessing low acid dissociation constants. They found the as-prepared nanostructured samples show bright structural colors and unique optical properties[31]. Dint can be tunable in the range of 645–884 nm by controlling the electrolyte concentration or anodization voltage These findings open a new way to explore large period PAA films, and are very helpful to develop new surface coating for aluminum and fabricate advanced chemical and biological sensors. The Janus anodization process is potential to become an alternative technology for traditional top-down nanofabrication technologies to efficiently and controllably fabricate microstructure or hybrid micro- and nanostructure with novel properties[26,27,28,29,30]
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