ng MWIL1 = 338.42 g/mole and ρIL1 = 1.19 g/mL, it turns outnox/nIL1 ~ 1.7. This value is of the same order of magnitude as for IL2. Anyway even forc = 0.1%, no APA was obtained in the case of IL1, such that this negative result can only be assigned to an inherent chemical difference between the interaction of the two ILs with the oxalic acid.
IL Aided Characterization and Processing
After using the IL2 as an additive in anodization, we have also tried to take advantage of its properties in the characterization of the system. In Fig. 2b, the liquid CAs θ measured on the APA surfaces described in Figs. 12a have been reported. The filled squares represent the CAs obtained with DI-water as the wetting phase. One can see that all the respective APA films look rather hydrophilic (θwater < 90°). Most samples showed quite similar values (θwater ~ 29°), whereas only the sample with smaller pores showed a significantly higher θwater. The reason is probably that for that sample the pores were too small to be filled by the water, and the drop was actually sitting on a mixed APA–air interface . In practical terms, the water “probe” did not allow for high enough resolution to sense the smallest APA pores. A similar resolution limit affected also the topographic measurements by AFM, as observed in the RMS roughness plot of Fig. 2a (see the previous subsection for discussion). In Fig. 2b, the open squares report instead the CAs obtained with a c = 10% v/v solution of IL2 in the DI-water wetting phase, θIL2. In this case, the CA values were all quite similar to each other, and lower than for the bare water, θIL2 ~ 16°. This means that the IL2 solution had higher wetting power than water, and could wet even the smallest APA pores. Actually a similar behavior is expected from any IL, which should work such as a highly polar solvent that can easily penetrate voids of a few nanometer diameter only . Therefore, IL2 can also be used successfully for this kind of characterization of the porous APA surface morphology.
The effect of addition of ILs into an oxalic acid aqueous solution commonly used for the fabrication of APA has been investigated. Two different ILs have been used for the first time as additives in this anodization process. By adding one of them, namely 1-butyl-3-methylimidazolium tetrafluoborate, in an approximately 1:1 molar ratio with the solution acid, and properly tuning the current density, we could obtain a growth rate of APA of 1.1 μm/min. This growth rate is comparable to the value normally obtained in the industrially applied HA conditions, but has been obtained in MA conditions in our case. Therefore, our process should make it possible to obtain thick APA layers in comparatively short times (order of few hours) and with ordered pore arrays also on the outer surface, after two-step anodization in the appropriateV range. The high-anodization current in itself does not guarantee fast APA growth, as demonstrated when the other IL was used as the EL additive. Therefore, a better understanding of the chemical mechanisms underlying the observed increase in growth rate has to be pursued, and is currently the subject of further research activity in our group. However, the presently reported preliminary results hold promise for the development of a technologically viable procedure for the fast growth of APA. In this application perspective, the possible use of ILs in characterization of the porous film and in its subsequent processing has also been explored. As a result, the selected IL has been demonstrated to be useful also as a pore wall protection medium during pore opening of APA, a process step that is often taken when passing membranes are fabricated out of the supported porous surfaces.
The authors would like to thank Mr. Romeo Losso for providing the original idea of using ionic liquids in the preparation of anodic porous alumina and for the recommendations, thereafter, and Mr. Claudio Larosa for technical support and useful discussions on the topic.
- Masuda H, Fukuda K: Science. 1995, 268: 1466. ; COI number [1:CAS:52