# Three-Dimensional Anodic Aluminum Templates Via A Single-Step Pulsed Potential Method

Pedro Resende, Georges Hadziioannou, Guillaume Fleury

► To cite this version:

Pedro Resende, Georges Hadziioannou, Guillaume Fleury. Three-Dimensional Anodic Aluminum Templates Via A Single-Step Pulsed Potential Method. Materials Today Communications, 2024, 39, pp.109244. 10.1016/j.mtcomm.2024.109244. hal-04891596

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Submitted on 16 Jan 2025

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# Three-Dimensional Anodic Aluminum Templates Via A Single-Step Pulsed Potential Method

Pedro M. Resende,\* Georges Hadziioannou, Guillaume Fleury

Univ. Bordeaux, CNRS, Bordeaux INP, LCPO, UMR 5629, F-33600, Pessac, France

\* Corresponding Author. E-mail address: pdecamposres@u-bordeaux.fr

## Abstract

Recent advances in the fabrication of three-dimensional Anodic Aluminum Oxide templates have opened novel opportunities in the generation of photonic structures and material nanostructuration. So far these structures have been produced from the standard two-step pulsed potential anodization and modulated current density anodization approaches. However, their fabrication is slow, with the two-step approach relying on intermediate oxide removal steps that often use toxic etchants. In this work, we report the fabrication of 3D Anodic Aluminum Oxide templates from a single-step pulsed potential method, completely removing the need for intermediate etching steps, and decreasing total fabrication times to less than 24 hours. Structural coloration was also successfully demonstrated and analyzed in these templates, for 99.999% and 99.5% Al precursors, presenting the same morphology and optical functionality as templates produced from two-step processes.

Keywords: Template, Anodic Aluminum Oxide, 3D, Alloy, Anodization
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# 1. Introduction

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Anodic Aluminum Oxide (AAO) is a very well-known inorganic porous oxide grown through the electrochemical oxidation of aluminum. This porous oxide has earned multiple applications in industrial and scientific settings [1]. In the industry, it is used for metal finishing, for coloration from the infiltration of dyes, or as a conformal coating for corrosion and abrasion resistance. In the scientific front, the development of the two-step anodization process by Masuda and Fukuda in 1995 [2], broadened their functional scope by taking advantage of the nanopore self-ordering. The resulting well-ordered array of nanopores has been used in the fabrication of sensors [3]; photonic crystals and coloration [4-9]; drug delivery [10]; material nanostructuration [11-16] (nanowires, nanofibers, nanodots, nanomeshes, etc.); and the study of dimensional confinement of inorganic and organic materials.[15-20] Currently, well-ordered AAO templates can be produced from a vast collection of acidic solutions (sulfuric, oxalic, phosphoric, selenic, tartaric, malonic, among other acids) yielding hexagonally packed vertical nanopores with diameters ranging from below 10 nm up to 700 nm [1, 21, 22]. Further research on AAO fabrication led to the development of modulated nanopore arrays, and eventually to the fabrication of fully 3D architectures by generating interconnections between the vertical pores [22]. In parallel publications, Gösele [23, 24] and Losic [25] paved the way for the generation of 3D AAO architectures by combination of mild and hard anodization (MA and HA, respectively) regimes with different electrolytes. This approach was further refined by Martín et al. [26], using a pulsed potential profile in a sulfuric acid electrolyte. The combination of MA and HA regimes is used to generate periodic zones of high oxide solubility (due to oxide defects or impregnated electrolyte anionic impurities) that can later be chemically etched to form transversal nano-channels adjacent to the vertical nanopores. This structuration was shown to produce strong structural coloration, behaving as a distributed Bragg reflector (DBR), from the modulation of the refractive index. These works are at the cornerstone of a new class of 3D templates that could readily be employed in material nanostructuration.

Other approaches followed a current density based method. In 2011, Yisen and colleagues [27] reported on the fabrication of AAO presenting structural coloration by the application of an alternating square-pulse current density profile. Variations of this method were further explored in subsequent years by Santos et al. [28-30], which led to the fabrication of different AAOs with envisioned applications in structural coloration [31], as DBR sensors [32, 33], optical filters [29, 30, 34], photocatalysis [5], among other examples. The application of these structures as 3D AAO templates was later explored by Resende et al., combining the alternating current density profiles with the electrolyte etching effect to create transversal interconnecting planes.[35-37] The use of these templates in polymer infiltration resulted in 3D polymeric networks that
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behaved as DBRs, opening another route for the fabrication of photonic structures from template-assisted nanostructuration.

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The reported current density based 3D AAO templates have a clear advantage when compared to potential-based approaches, as their fabrication only requires a single step without additional post-processing unlike the potential-based case. However, the application of a current density profile (or even a standard galvanostatic anodization) requires a careful control of the effective surface area. This becomes an issue when dealing with aluminum substrates with arbitrary surface shapes, high surface roughness, or surface patterning, all of which greatly increase effective surface area. This main drawback could be addressed by using a potential-based alternative that yields 3D AAO templates with reduced fabrication times, less processing steps, and compatibility at an industrial scale.

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In this work, we demonstrate the fabrication of 3D AAO templates by direct application of a pulsed potential profile in a single anodization step. Limiting the number of anodization steps directly decreases total fabrication time while also bypassing the need to chemically remove the oxide that is typically grown in the two-step process. Most literature examples resort to different etching procedures, some of which with toxic chemicals such as the particular case of chromic oxide based etchants. Furthermore, we also demonstrate the application of the same procedure in low purity aluminum alloys, not only creating 3D morphologies but also obtaining structural coloration. These structures show that a photonic response can still be observed in these Al alloys despite the presence of oxide defects (voids and metal impurities) and the highly disordered nature of the resulting nanopores. This work brings 3D AAO templates another step closer to their wide fabrication, commercialization, and application, mainly in filtering and sensing, optical filters and reflectors, and material nanostructuration, with this last case being highly significant in the study and fabrication of new metamaterials.

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## 2. Experimental Section

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### 2.1. Materials

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Aluminum (99.999% and 99.5%) sheets (thickness of 0.5 mm) were purchased from Advent Research Materials Ltd. (Oxford, UK). Acetone (ACS Reagent), Cyclohexanone, and Triethyl Phosphate (TEP) were purchased from Sigma-Aldrich. Isopropanol and Ethanol (Absolute) were purchased from VWR Chemicals (France). H₂SO₄ (95-97%) and H₃PO₄ (85%) were purchased from Honeywell-Fluka (Germany). HClO₄ (70%) and HCl (37%) were purchased from Fisher Chemical (UK). CuCl₂ (anhydrous, 99%) was purchased from Acros Organics (Germany). Polycaprolactone (PCL, 3 mm
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granules) was purchased from Sigma-Aldrich. Poly (vinylidene-fluoride-*co*-trifluoroethylenes) [P(VDF-TrFE), 50% TrFE, powder] was kindly supplied by Arkema-Piezotech.

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## 2.2. Aluminum Pulsed Anodization

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Firstly, the aluminum sheets were cut into rectangular substrates of 2×4 cm. The Al substrates were cleaned under ultrasonic agitation in consecutive baths of acetone, isopropanol, and ethanol, for 6 minutes each. Sample electropolishing was then performed by immersing the Al substrates into a $HClO_4$ and ethanol solution (1:3 in V/V) and applying 20 V for 5 to 6 minutes. Strong stirring was applied (> 1 krpm), ensuring a uniform surface polishing. A Pt80/Pd20 wire mesh was used as counter-electrode (anode). After electropolishing, the substrates were rinsed with ethanol and deionized water, and left to dry. A Kapton® and Parafilm® mask was used to limit the active surface area for the anodization process, having a rectangular shape with 1.5×2.5 cm (3.75 cm²). The electrolyte did not react with the mask, and the electrolyte-substrate contact area was kept constant. The anodization followed the same approach as Martin *et al.* [26] for pulsed anodization in 0.3 M $H_2SO_4$, but resorting only to a single step. In our approach, the pulsed anodization is directly performed on the polished Al substrates. This potential pulsed method in $H_2SO_4$ is based on the periodic application of 2 s pulses of 33 V (HA regime), after which the potential is lowered to the 25 V (MA regime). This is shown in Figs. 1a and b. Several samples were prepared, with three different inter-pulse periodicities (180, 240 and 360 s). Prior to the pulsing regime, the oxide was grown at 25 V for 20 min, generating a porous oxide layer that confers protection from breakdown. A minimum of 150 pulses were applied. The same process was applied for the 99.5% Al substrates, except for the initial layer of oxide. For these samples, the initial layers were grown for 1 up to 4 hours. The total anodization times varied from 18 to 22 hours, depending on the inter-pulse period and total number of pulses. All anodization processes were performed in a 0.3 M $H_2SO_4$ electrolyte, at -1 °C, under constant stirring (~ 450 rpm), with a base potential of 25 V (MA) and a pulse potential of 33 V (HA). After the anodization, the samples were cleaned in deionized water, dried, and stored.

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## 2.3. Template Post-Fabrication Processing

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Post-fabrication etching was performed to obtain the 3D AAO templates. Firstly, the remaining Al substrate was removed with an HCl and $CuCl_2$ aqueous solution (27.7%wt. HCl 37%, 1.7%wt. $CuCl_2$). Secondly, the AAO barrier layer was removed with a 10%wt. $H_3PO_4$ solution, at room temperature, lasting around 30 min for AAO fabricated from 99.999% Al, and a maximum of 10 min for those obtained from 99.5% Al. A last pore etching process was performed to obtain the final
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3D AAO structure. The pore etching was done with a 5%wt. H₃PO₄ solution, at 30 °C, lasting at least 30 min for 99.999%
Al AAOs and around 27 min (or lower) for 99.5% Al AAOs. For 99.999% Al samples, coloration usually becomes visible
after 30 min of etching time. After the etching process, the samples were cleaned in deionized water and left to dry.

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## 2.4. Polymer Infiltration

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P(VDF-TrFE) and PCL were infiltrated into different 3D AAO templates. PCL was melt infiltrated by placing previously prepared PCL thin films on top of a 3D AAO, at 150 °C, for 4 hours, on a hot-plate. P(VDF-TrFE) was infiltrated inside 3D AAO templates by solution and melt infiltration. Solution infiltration was performed by immersing the 3D AAO template inside a 2 wt.% P(VDF-TrFE) solution in TEP for 24 hours, after which the samples were dried on a hot plate at 90 °C for 3 hours. P(VDF-TrFE) melt infiltration followed the same procedure as with PCL, at a temperature of 200 °C, for 6 hours, using previously prepared thin films. All samples were then left to cool down naturally to room temperature.

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PCL thin films were prepared by drop-casting 10 wt.%. PCL solutions in cyclohexanone on top of glass substrates and dried at 90 °C for 2 hours on a hot plate. P(VDF-TrFE) thin films for melt infiltration were prepared from 10 wt.% P(VDF-TrFE) solutions in TEP, by drop-casting and drying at 90 °C for 3 hours.

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## 2.5. Morphological Characterization

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Morphological characterization was performed with TEM and SEM. The TEM was performed with a Hitachi H7650 (equipped with a Gatan ORIUS 11MPx camera), under 80 kV. Specimens for TEM were prepared by immibing the different samples in an epoxy resin, followed by sample delamination (average thickness of 150 nm) with an ultramicrotomy system from Leica (UC-T Leica) using a diamond blade. The cutting direction was perpendicular to the template nanopores. SEM was performed with a JEOL 7800-E PRIME (Jeol Ltd., Japan) as well as a GeminiSEM300 FESEM (Zeiss). Observation was performed under low current (order of pA) and voltage settings (maximum 1 kV) to minimize sample charging and beam-induced damage (polymer structures).

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## 2.6. Optical Characterization

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Spectrophotometry was performed with a UV-Vis spectrophotometer from Shimadzu (Shimadzu UV2600), with an integration sphere for reflectance measurements. The spectra were recorded for the range of 300 up to 1000 nm. Several transmittance measurements were also performed for a range of 200 to 1200 nm, with a Cary 5000 Series, UV-Vis-NIR
spectrophotometer. The visibility of the coloration depends on the contrast between the background signal and the maximum of the stop-band. The transmittance contrast was estimated according to the following equation for the Weber contrast ($c_W$):

$$c_W = \frac{I - I_b}{I_b}$$

Where $I$ is the maximum intensity of the stop-band and $I_b$ is the background intensity. In this calculation we have used the transmittance, $t$, values, with $I - I_b$ representing the relative transmittance with respect to the background transmittance, $\Delta t$. As the transmittance (and reflectance) of AAO templates varies with the wavelength, we have estimated the average background transmittance, $t_b$, around the stop-band maxima from a spectral baseline fitting with adjacent averaging. The corrected equation is:

$$c_W^{AAO} = \frac{|t_{stop-band} - \langle t_b \rangle|}{\langle t_b \rangle}$$

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## 3. Results and Discussion

The fabrication of single-step 3D AAO templates followed a shorter approach to that used by Martín et al. [26] The two-step anodization process for the fabrication of 3D AAO templates is shown in Fig. 1a. For the two-step approach (red box, Fig. 1a), the first-step consists on performing a potentiostatic anodization under mild anodization conditions (MA regime). This step is usually performed for 24 h, at 25 V for 0.3 M sulfuric acid electrolytes. The grown oxide is then removed, exposing the pattern aluminum surface from the well-ordered and hexagonally packed nanopores. These surface indentations serve as nucleation sites for nanopore grown in the second anodization step. To obtained 3D AAO templates, a pulsed potential profile is applied during the second anodization step (such as the one shown in Fig. 1b), with a base voltage of 25 V and a pulse voltage of 33 V. The resulting 3D AAO is then obtained after a series of chemical post-fabrication treatments (see experimental section) that include the removal of the remaining Al substrate, the removal of the bottom barrier of the nanopores, and a final pore etching treatment (also referred to pore widening). In particular, the last chemical etching step leads to the formation of transversal openings in a network-like architecture with hexagonal symmetry. However, the working principle behind this process is independent of nanopore ordering, as the transversal channels are formed due to oxide solubility gradients from chemical/lattice defects, such as impurity impregnation (e.g.,
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sulfate anions) or vacancies/voids, during the HA pulses [1, 26]. Following this premise, a single-step anodization can be
performed that directly applies a varying potential signal without the need for a first anodization at MA regimes. This is
also illustrated in Fig. 1a (green box), where the pulsed regime is applied directly to the aluminum substrates. A short
potentiostatic is included in the applied potential profile (as shown in Fig. 1b), generating an initial layer of porous oxide
that acts as protection against dielectric breakdown during pulse application. To demonstrate this approach, several single-
step pulsed anodizations were performed, with different inter-pulse periods (t<sub>period</sub> of 180, 240, and 360 s). The choice of
periodicity is based on previously reported works on these 3D AAO templates, that yielded structural coloration. The
presence of a photonic response is an indication of the successful generation of the 3D architecture. This procedure was
applied to high purity aluminum (99.999%, typically employed in the literature) but also to a lower purity alloy (99.5%),
studying the possibility to generate the same morphologies with a more affordable alternative to ultra-high purity precursors.
Previous literature works have also investigated the fabrication of AAO in high and low purity Al precursors, such as the
work from Zaraska *et al.* [38] analyzing oxide growth rates and current transients under potentiostatic conditions and
obtaining similar transient tendencies for the considered alloys. The anodizations in this work were performed with a
minimum number of pulses of 150, and following the same conditions for 99.999% and 99.5% Al. For 99.5% Al, an initial
thicker layer of oxide was grown (up to 4 hours) compared to only 20 min for 99.999% Al, decreasing the risk of oxide
breakdown during the pulsing regime. The importance of this protective layer will be further discussed below.
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Schema of the anodization process and images of the produced samples. (a) Schema comparing the two-step (red) and single-step (green) anodization approaches. In the single-step, the pulsed anodization regime is directly applied prior to a short potentiostatic regime. (b) Example of the pulsed potential profile that is applied for the fabrication of the 3D AAO templates. (c) As-prepared 3D AAO from high purity Al (99.999%). (d) As-prepared 3D AAO from low purity Al (99.5%). (e) As-prepared 3D AAO from 99.5% Al, presenting oxide breakdown (dark and light-gray areas). (f) 3D AAO after post-fabrication chemical etching, produced from 99.999% and 99.5% Al for three different $t_{period}$: 180, 240, and 360 s. Sample etching time is also shown for each sample.</figcaption></figure>

## 3.1. Macroscopic Structure

After the anodization process, differences in the macroscopic appearance of the oxide can be immediately observed depending on the purity of the precursor metal. Figs. 1a, b and c show examples of as-prepared 3D AAOs (prior to chemical etching) from 99.999% Al (a) and 99.5% Al [(b) and (c)]. The anodized regions are located inside the dashed blue lines. For the case of 99.999% Al (a), the grown oxide presents a yellowish coloration compared to the blue hue that is usually found in standard anodizations. This change in coloration can be associated to a higher amount of sulfate anions present in the oxide because of the HA pulses. The oxide grown with 99.5% Al [(b) and (c)] presents an opaque gray color due to the
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presence of impurities in the metal that darken the coloration and act as light scattering centers. Lastly, Fig. 3c shows a particular example of oxide breakdown for an anodization with 99.5% Al, presenting small dark and light gray spots primarily at the edges and corners of the substrate. These surface defects were observed to be less frequent when increasing the thickness of the first layer of oxide grown under MA conditions (25 V for this case), prior to the application of potential pulses. After the anodization, several chemical etching steps were performed to obtain the 3D AAO structure: (i) the removal of the remaining Al substrates; (ii) the dissolution of the barrier layer, corresponding to the bottom of the grown pores; and (iii) the etching of the internal structure of the template, gradually opening the transversal pores (pore widening). The duration of these chemical steps was different for the two types of Al, with the 99.5% Al presenting faster etching rates. This was particularly significant during the removal of the barrier layer, with an average duration of only 7 minutes for 99.5% Al compared to 30 minutes for 99.999% Al (at room temperature). The prolonged contact between the Al etchant and the oxide further decreased the barrier layer removal times due to partial dissolution of the oxide by the HCl-based etchant. The etching times observed for the last step (iii) were very consistent between the two types of samples, with visible coloration after 30 min. This may suggest that despite the higher solubility of the 99.5% Al oxide, other mechanisms may influence the chemical process (such as trapped air or ion depletion) and decrease the etching rates. However, high purity samples could endure longer etching times (up to 50 min or more) before structural collapse, while low purity samples would lose structural integrity after 30 min. The resulting samples are shown in Fig. 3d (observed approximately at 90° from the surface), organized as a function of the applied inter-pulse time period, $t_{period}$, and the type of Al used. All the samples presented a strong structural coloration, with the exception of the sample with $t_{period}$ of 180 s produced from 99.5% Al, that presented a translucent gray color. Changes in the apparent brightness or reflectivity of the surface can be explained by the small curvature of the used substrates. A coloration shift is observed for all the samples with increasing $t_{period}$. An increase in $t_{period}$ results in an increase of the spatial periodicity (distance between transversal pores) of the templates, shifting the response towards larger wavelengths. Following this, we can speculate that the response from sample $t_{period}$ 180 s and 99.5% Al was potentially located in the UV part of the spectrum. A similar discrepancy can also be seen for the samples produced from 99.999% Al, starting with a purple/dark-blue coloration, followed by orange and then another color tone around dark blue. The same argument applies here, with the observed coloration corresponding to a high-order reflection, with the first-order reflection located outside the visible range (near-IR region). The behavior of these systems can be understood through the Bragg-Snell law [37]:
$$n\lambda = 2h\sqrt{n_{eff}^2 - n_{air}^2\sin^2\theta}$$

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Where $n_{eff}$ represents the layer effective refractive index; $n_{air}$ is the refractive index of air (ca. 1); $h$ is the thickness of the layer; $n$ is an integer that represents the reflection order; $\lambda$ is the wavelength of the reflected/incident light; and $\theta$ is the angle of incidence with respect to the normal of the surface.

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From this expression, it is evident that the reflected wavelength $\lambda$ has a direct linear dependence with the thickness of the layers, here represented by the spatial periodicity of the templates. It follows that the two particular cases of 99.999% Al with $t_{period}$ 360 s and 99.5% Al with $t_{period}$ 180 s most likely correspond to cases in which the first order reflections are outside the visible range.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Microstructure characterization and oxide formation. (a) TEM micrographs of samples with *t*<sub>period</sub> from 180 to 360 s for the two aluminum grades, showing the spatial periodicity. (b) Plot of the sample spatial periodicity as a function of *t*<sub>period</sub>, and the estimated growth rates, *g*<sub>rate</sub>. (c) Transversal pore size as a function of *t*<sub>period</sub>. (d) SEM micrographs showing voids and defects from the presence of metallic impurities in 99.5% Al. (e) Close-up SEM micrograph of a cross-section cut from a 99.5% Al template presenting the transversal pores connecting adjacent vertical pores with a focus on over-etched and uneven pores. (f) Scheme illustrating the chemical reactions that take place during AAO formation, and the potential reactions at the impurity-electrolyte interface. (g) Circuit analogy of the AAO and impurity system depicting charge/current deviation in the Al alloy. (h) Scheme relating the pulsed anodization profile with anion-concentration variations inside the oxide responsible for the transversal nanopore generation.</figcaption></figure>
## 3.2. Microstructure Characterization

TEM and SEM were used to analyze the inner structure of the templates. The obtained micrographs are shown in Fig. 2, from which the template spatial periodicity and transversal pore size were extracted. Fig. 2a displays TEM micrographs for all the considered $t_{period}$ (180, 240, and 360 s), for 99.999% and 99.5% Al. The spatial periodicity is seen to increase with $t_{period}$. Furthermore, for the same $t_{period}$, the spatial periodicity is smaller in the Al alloy samples. The transversal pores that form the transversal interconnections in this type of 3D AAOs are also visible for all the cases. This supports the earlier hypothesis that some samples present reflections outside the visible range. The sample spatial periodicity and average pore size are plotted in Figs. 2b and 2c, respectively. The spatial periodicity increases approximately linearly with $t_{period}$, with 99.999% Al presenting longer spatial periods (from ca. 140 to 310 nm) and larger variations with $t_{period}$. For the Al alloy, the spatial periods range from ca. 50 to 160 nm, considerably lower than the previous case. The growth rates, $g_{rate}$, can be estimated from the measured data yielding 0.95 nm/s and 0.66 nm/s for 99.999% and 99.5% Al, respectively. These values are more commonly reported in µm/h, corresponding to ca. 3.4 and 2.4 µm/h, which are well in agreement with previous reported oxide growth rates for high and low purity Al alloys [1, 38, 39]. It is also important to note that growth rates can vary from setup to setup due to a variety of conditions: bath or substrate temperature variations, local concentration variations, stirring, or size and distance of counter-electrode. Variations in growth rate will affect the photonic response of these templates and are expected to be observed between different setups. Sample fabrication calibration should be performed after assembling the experimental setup. Fig. 2c shows the transversal pore size obtained from the TEM micrographs. The sizes obtained for high purity Al are fairly consistent. The duration of the HA pulses was kept constant at 2 s, regardless of the employed $t_{period}$, resulting in consistent and repeatable pore size distributions. Small variations are due to subsequent etching steps that may result in slightly enlarged (or reduced) sizes from prolonged (or short) etching times. Despite this, the trend appears to be different for the Al alloy, initially presenting smaller transversal pore sizes that appear to increase with $t_{period}$. Considering that these samples were grown under the same conditions (pulse duration, range of time periods, electrolyte, temperature, etc.), it should be expected that such variations are due to the nature of the used alloy rather than variations on the employed method. Figs. 2d and 2e show SEM micrographs of cross-section cuts of another template fabricated from 99.5% Al. The presence of large voids can be seen in Fig. 2d spanning 1 to 2 µm, and in some cases (not shown) up to 5 µm. Previous works on the anodization of Al alloys have shown similar structures that are formed
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from the gradual dissolution of impurities present within the metal [31, 40, 41]. During Al oxidation these impurity grains become exposed to the electrolyte and act as low resistance bridges. These low resistance pathways diverge current from the aluminum-oxide interface lowering the oxide growth rate. This is the case for the used Al alloy, presenting a lower growth rate. The chemical reactions responsible for the generation of the Al oxide are illustrated in Fig. 2f. Several reactions take place during the steady-state growth of the nanopores [1, 22, 42, 43]. These reactions take place locally around the electrolyte-oxide-aluminum interface and are limited by various factors, such as ion migration or ion depletion. The presence of metallic impurities decreases the formation and migration of Al³⁺ cations by disrupting the electrolyte-oxide-aluminum interfaces. At the same time, metallic impurities also reduce interface resistance and diverge current to the impurity-electrolyte interface by creating regions with more intense electric fields. The electrolyte chemical species undergo several reactions at the impurity-electrolyte interface, gradually dissolving the metallic impurity potentially through the formation of soluble oxides, hydroxides, and sulfates. Besides explaining the reduced growth rate, this process can also explain the more frequent occurrence of oxide breakdown during the pulsed anodization of the aluminum alloy. The sudden increase in the applied potential during the HA pulse greatly increases the current that flows through the electrochemical system. The low resistance pathways formed by these impurities absorb most of the surplus charge during a HA pulse, as illustrated in Fig. 2g, inducing local electric breakdown of the oxide. Because most of the current passes through these low resistance bridges, only a smaller fraction actively participates in the growth of the higher solubility oxide responsible for the formation of the transversal pores. This mechanism explains the apparent variation of the transversal pore size with the applied interpulse time period. As *t*<sub>period</sub> increases, the pore sizes approach the values observed for high purity aluminum. For this reason, it becomes paramount to avoid oxide breakdown by limiting the current that passes through the system or by growing a protective oxide layer through anodization under MA regimes prior to the application of potential pulses. Reducing the pulse frequency (increasing *t*<sub>period</sub>) was also seen to reduce breakdown probability as a thicker layer of oxide is allowed to grow between pulses. It should also be mentioned that impurity grain size should affect the breakdown probability. Although not explored in this work, larger grains may further increase breakdown probability. In this case, a longer pre-pulsing MA regime might be required (and longer *t*<sub>period</sub>) to balance this effect and reduce occurrence of electric breakdown. This approach was used for the fabrication of the 99.5% Al samples, initiating anodization under MA regimes for longer periods of time (up to 4 h, compared to only 20 min for high purity Al) before the pulsing regime. Fig. 2e shows a close-up cross-section of the transversal nanopores for an Al alloy sample. The transversal pores can be well resolved, forming the different
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interconnections between the adjacent vertical pores. The formation mechanism of these transversal nanopores is illustrated in Fig. 2h. When an HA pulse is applied, it generates a lower purity oxide, typically containing higher concentrations of electrolyte anions and defects. These areas are later responsible for the formation of the transversal pores through chemical etching. Finally, a considerable variation on the transversal pore sizes is observed as shown in Fig. 2e, which increases the deviation on size distribution (see Fig. 2c). This may be a result of uneven chemical etching in addition to the already discussed phenomena (diverged current during the HA pulse). Additional openings along the vertical nanopores can also be resolved arising from a prolonged chemical etching. From an optical point of view, the presence of additional defects in the oxide decreases the optical response of the system (disrupting refractive index modulation and contrast) and increases light scattering. Nevertheless, the presence of additional openings and branching may result beneficial in material nanostructuration, increasing the number of different interconnections.

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<figure><img src="image_4.png" /></figure>

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<caption>**Fig. 3.** Optical characterization of the produced templates. (a) Reflectance spectra of the 99.999% Al templates. (b) Reflectance spectra of the Al alloy (99.5%) templates. (c) Measured first-order wavelength maxima, $\lambda_{max}$, as a function of the applied $t_{period}$, presenting a linear tendency. (d) Comparison of the reflectance spectra of a single- and two-step 3D AAO from 99.999% Al (top graph), and as-prepared samples prior to chemical etching (bottom graph) showing small reflection peaks. (e) Transmittance spectra as a function of etching time for a single-step 3D AAO (150 pulses, 360 s) and three images</caption>
at different etching times. (f) Transmittance spectra as a function of etching time for a two-step 3D AAO (235 pulses, 360 s) and three images at different etching times. (g) Band Position, width, and contrast obtained from the data in (e) and (f) for the second-order maxima (visible range). Empty (□, ◦, and ▷) and Full (■, ◆, and ▶) symbols refer to the single-step and two-step templates, respectively.

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### 3.3. Optical Response

The optical response of the templates was studied with spectrophotometry, measuring the reflectance and the transmittance of different samples at various stages. The reflectance of the high and low purity Al samples is shown in Fig. 3a and b, covering the considered $t_{period}$. These samples were chemically etched after fabrication, with a pore etching time around 30 min. For the case of 99.999% Al, a shift of the reflectance maximum ($\lambda_{max}$) was detected with increasing $t_{period}$, starting around 465 nm (for 180 s) to 856 nm (for 360 s). Second-order maxima are also visible for the samples with $t_{period}$ of 240 and 360 s, located approximately at half of the first-order reflection (at 307 and 437 nm respectively). The sample with $t_{period}$ of 360 s presents a first-order reflection within the first part of the near-IR confirming that the observed coloration arises from the second-order maximum at 437 nm. The same tendency is observed for the Al alloy samples (Fig. 3b), albeit with much weaker reflections. No reflection peak was detected for a $t_{period}$ of 180 s. This is not unexpected considering the low reflectance of these bands, the high diffusivity of the Al alloy oxide, and the small transversal pore size, with the latter decreasing refractive index contrast (air to oxide). For 99.999% Al the maxima of the recorded spectra also appear to be composed by different peaks. This can be attributed to the post-fabrication chemical etching, which can create artifacts such as band broadening or appearance of shoulders due to uneven chemical etching. A careful control of the fabrication conditions is required, especially for low periodicities, to achieve the desired optical response. From the reflectance maxima (first-order), a linear relationship was extracted (see Fig. 3c) between the applied $t_{period}$ and the obtained $\lambda_{max}$ (for 99.999% Al, $R^2 > 0.99$).

To establish a comparison between the two-step and the single-step (one-step) process discussed in this work, we have also prepared additional two-step templates with high purity Al, measuring and comparing the optical responses. This is shown in Fig. 3d, where the reflectance spectra are plotted (top graph) for a one-step and a two-step samples. The samples show similar maxima and reflectance, independently of being fabricated with a single or double anodization process. The bottom graph shows reflectance spectra for several 1-step samples immediately after fabrication (no post-processing). Interestingly, we observed that the quality of the anodization process can be assessed from these spectra, as small maxima can already be
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detected for all cases (with or without oxide breakdown - BD in Fig. 3d - and with Al alloy). For the case of the sample presenting oxide breakdown, several additional peaks can be detected suggesting the pulsed-anodization process was disrupted after breakdown, changing the spatial periodicity of the sample due to changes in oxide growth rate. The information in this spectra was never reported or discussed in previous works dealing with potential pulsed anodization methods, and constitutes a fast method to verify the quality of the produced samples without resorting to their chemical post-processing. In addition, the fact that diffraction peaks are detected post-fabrication without a visible morphological modulation favors the theory that chemical gradients or structural defects are formed during HA pulses responsible for the selective dissolution of oxide during the subsequent treatments. These gradients provide a significant photonic contrast that renders them detectable with a simple reflectance measurement.

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Finally, the evolution of the transmittance spectra with etching time was assessed for samples produced through a single- and two-step process (same $t_{period}$). Fig. 3e shows the transmittance spectra for a single-step sample, with 150 pulses. The stop-bands can already be detected at short etching times of 10 min (up to 4 orders of reflection can be resolved), gradually broadening and shifting to lower wavelengths with prolonged etching times. The most significant loss of transmittance occurs for an etching time of 50 min, after which the samples lose structural integrity and break. Images are shown for three different etching times: 35, 39, and 50 min. Fig. 3f shows the same analysis but for a two-step sample, with 235 pulses. The same tendency is observed, with multiple stop-bands detected for short etching times, gradually broadening and blue-shifting. The most significant loss of transmittance takes place at a later stage, around 55 min of chemical etching, after which the oxide loses structural integrity. Images are also shown for the same etching times as in Fig. 3e. The plot in Fig. 3g contains the data extracted from the second-order (always located within the visible range) stop-band detected in the transmittance spectra as a function of etch time (empty symbols [□, ◦, and ▷] for single-step; full symbols [■, ◆, and ▶] for two-step). The two types of samples present stop-band blue-shift and band broadening with etching time, with the two-step sample having a more significant band broadening. The visibility of the coloration depends on the contrast between the band and the background signal. We have estimated the transmittance contrast for the different spectra, also shown in Fig. 3g. The transmittance contrast presents the same tendency for the two cases, but reaches values closer to unity for the single-step method. As the oxide modulation process is the same for the two cases, the observed changes can only be related to the total number of pulses applied for each case. The total thickness of the samples is dependent on the total number of pulses, meaning that the two-step sample with 235 pulses is substantially thicker than the single-step sample prepared with only
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150 pulses. The thickness of the oxide will impact the optical response of the sample, not only by decreasing sample transmittance but also by causing uneven nanopore etching from etchant depletion. This effect broadens the stop-band and decreases the transmittance contrast. This is not an issue for applications that require broader bands, but limits the achievable contrasts. Thinner samples (low number of pulses) have the advantage of presenting higher contrasts due to the sharper stop-bands, but have low structural integrity around the transmittance contrast maxima due to prolonged etching and pore collapse.

<figure><img src="image_5.png" /></figure>

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**Fig. 4.** Polymer infiltration into single-step 3D AAO templates. (a) P(VDF-TrFE) melt infiltration inside a 99.999% Al 3D template (150 pulses), with several nanowires emanating from the transversal and horizontal channels. (b) PCL melt infiltration inside a 99.999% Al 3D template with 235 pulses. (c) Solution infiltration of P(VDF-TrFE) into a 99.5% Al alloy 3D template, where the different nanowires were observed to collapse, forming a nanowire agglomerate on the surface.

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## 3.4 Material Nanostructuration

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A frequent application of AAO templates is material nanostructuration, producing nanowires and nanotubes, and more recently 3D network-like architectures from organic and inorganic materials. We have infiltrated polymeric materials into the produced single-step 3D AAO templates, demonstrating that these templates (from high purity and Al alloy) are equally suitable for material nanostructuration. Fig. 4 shows several SEM micrographs of infiltrated single-step 3D AAO templates. The template shown in Fig. 4a was infiltrated with poly (vinylidene-fluoride-co-trifluoroethylene) [P(VDF-TrFE), 50% TrFE], through melt infiltration, where different polymer nanowires and nanofibers can be distinguished, completely filling the vertical and horizontal channels. The inset in Fig. 4a is focused on the transversal interconnection zone, where multiple nanowires can be seen emanating from the different channels. The same method was used to infiltrate polycaprolactone (PCL), seen in Fig. 4b, with several visible nanowires along the AAO fracture, as well as inside the vertical and horizontal pores. Lastly, an Al alloy template was also used, shown in Fig. 4c, where P(VDF-TrFE) was infiltration through a 2 wt.% solution. The resulting polymer network detached from the exposed nanopores and collapsed due to surface tension, forming
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a nanowire/nanofiber bundle on the surface of the cross-section. Several traces of over-etched nanopores can also be seen interlaced with the collapsed fibers. As a result of the low solution concentration, the obtained nanowires were observed to be much thinner with incomplete filling of the template. The resulting polymeric networks show that single-step AAO templates can also be used for the fabrication of nanostructured polymer networks, which can find applications in the study of polymer confinement behavior inside 3D structures; membranes and filters; or scaffoldings for cell culture.

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## 4. Conclusion

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In this work, it was shown that a single-step pulsed potential anodization process is suitable for the fabrication of 3D AAO templates, with high and low purity Al precursors (99.999% and 99.5%). Structural coloration was attainable with this process for both types of aluminum purity, irrespective of nanopore ordering. This is the first time that coloration is reported for aluminum alloys using the pulsed potential approach. We have also rationalized the variations of oxide growth rate and transversal pore size distribution for Al alloys and how these can be mitigated through the growth of protective layers or extended pulse periodicity.

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An in-depth analysis of the optical response demonstrated the possibility of using simple reflectance measurements for anodization quality control by revealing the presence of local reflectance maxima prior to chemical post-fabrication treatments. In addition, the analysis of the transmittance spectra as a function of the oxide etching time has shown that thinner samples with low number of pulses can achieve high coloration contrast and narrower stop-bands; while thicker samples with a larger number of pulses lose coloration contrast due to stop-band broadening from etchant depletion. Samples produced with the single- or two-step anodization processes did not present differences in their optical responses. Lastly, material nanostructuration was also demonstrated, using single-step anodization samples from 99.999% and 99.5% Al, and using polymer melt and solution infiltration. Melt-infiltration also produced samples with better filling ratios, as seen from SEM characterization.

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## Declaration of Competing Interest

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The authors declare no conflicts of interest, or any known competing financial interests or personal relationships that could have appeared to influence this work.
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Acknowledgements

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The authors thank Arkema-Piezotech (France) for providing the materials used in the study. Financial support from the
Industrial Chair SMILE within the Grant Agreement No. ANR-19-CHIN-0002.

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ORCID IDs

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Pedro M. Resende orcid.org/0000-0001-7854-2297

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Georges Hadziioannou orcid.org/0000-0002-7377-6040

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Guillaume Fleury orcid.org/0000-0003-0779-191X

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