{"paper_id":"0270a67d-c5db-4afc-8b39-ff11704bb653","body_text":"Nanopore Architectures in Anodic Aluminum Oxide: Effects of Anodization Voltage and Time on Planar and Non-Planar Aluminum Substrates | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Nanopore Architectures in Anodic Aluminum Oxide: Effects of Anodization Voltage and Time on Planar and Non-Planar Aluminum Substrates Mawar Hasyikin Abu Seman, Nor Izzati Gati, Abdul Hadi Mahmud, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7792391/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 23 Feb, 2026 Read the published version in Journal of Porous Materials → Version 1 posted 20 You are reading this latest preprint version Abstract This research investigates the effect of voltage changes and anodization duration on the nanopore architecture of anodic aluminum oxide (AAO) across various aluminum (Al) substrates, including both planar and non-planar forms. After a two-step anodization, planar Al substrates developed a porous layer limited to their flat surface. In contrast, the Al wire, due to its curved shape, experienced an intensified local electric field, resulting in thicker oxide layers around its circumference. The most significant effect was observed in hollow Al tubes, where nanoporous layers formed simultaneously on both the inner and outer surfaces. This dual-surface anodization significantly increased the effective surface area and produced the thickest oxide layers among all tested substrates. Field-emission scanning electron microscopy was used to analyze the morphology of the AAO. The findings indicated a direct relationship between the applied voltage and the diameter of AAO pores, with pore sizes increasing from 30.0 to 150.0 nm for planar substrates and from 30.0 to 220.0 nm for non-planar substrates as the voltage increased from 40 V to 100 V. AAO thickness ranged from 12.7 to 47.0 µm for planar substrates and from 14.0 to 60.0 µm for non-planar substrates. The surface structure of the Al substrates also influenced the distribution of AAO pore diameters. The dual-layer AAO on Al tubes exhibited larger pores and greater interpore distances, which can be attributed to differences in oxide growth direction and electrochemical field distribution. These findings offer valuable guidance for the engineering of non-planar AAO materials for diverse applications. AAO Al wire Al hollow tube AAO nanostructure two-step anodization Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1.0 Introduction Anodic aluminum oxide (AAO) refers to a nanoporous structure formed through the electrochemical oxidation process, also known as anodization of aluminum. This process creates a highly ordered arrangement of nanopores, resembling a honeycomb structure. Significant advancements in this field emerged in 1995 with the introduction of a two-step anodization process by Masuda and Fukuda [1]. They proposed a pre-texturing approach to fabricate porous AAO membranes [2]. The anodization process has become an appealing option due to its unique ability to offer precise control over the morphology and thickness of the AAO porous layer. This method has proven to be highly repeatable, offering advantages such as cost-effectiveness, rapid deposition times, ease of operation, and scalability [3]. These attributes make anodization a compelling choice for the production of AAO, enabling researchers to harness its unique properties for a wide range of applications, not only limited as functional nanomaterials to gas separation [4, 5], sensing [6, 7], and energy storage [8, 9], but the usage of the AAO can also be extended as a template for creating nanostructures like nanowires and nanotubes [10, 11]. The fabrication of AAO depends on the relationship between anodization conditions and the resulting AAO structure. Precise control over the size and spacing of these nanopores is achieved by adjusting anodization variables. Given that previous studies have extensively explored factors such as electrolyte composition, temperature, and current density [12-15], this study narrows its focus to the impact of anodization voltage and time on AAO formation in planar and non-planar Al substrates. Understanding how these two key parameters influence pore structure and oxide growth can further optimize AAO fabrication for various applications. In particular, the growth process of porous AAO involves a balance between the reactions that occur at both the anode and cathode. The primary response at the anode consists of the oxidation of aluminum to form aluminum oxide (Al 2 O 3 ). The general electrochemical anodizing processes can be represented as equations below: Oxidation of Al at the anode: 2Al + 3H 2 O ® Al 2 O 3 + 6H + + 6e (1) Reduction of H + at the cathode: 6H + + 6e ® 3H 2 (2) Overall reaction: 2Al + 3H 2 O ® Al 2 O 3 + 3H 2 (3) The most remarkable feature of AAO lies in its nanoporous structures, characterized by highly ordered pores or channels that form a quasi-hexagonal, close-packed arrangement. AAO can be engineered with specific nanostructures, encompassing pore size, interpore distance, pore density, and oxide layer thickness. Typically, AAO is configured as a bulky membrane attached to an Al foil substrate, maintaining a flat profile [16]. However, conventional planar AAO membranes have inherent limitations, particularly their brittleness and difficulties in handling. Moreover, they provide only a one-dimensional surface for functionalization. It makes non-planar AAO structures, such as wires and tubes, an attractive alternative to explore. While adopting non-planar configurations does not eliminate brittleness, the presence of the underlying metallic substrate provides additional mechanical support, which helps to alleviate some of the handling challenges. Additionally, non-planar AAO offers multidimensional surfaces for functionalization, thereby expanding the range of potential applications. The proposed schematic formation of AAO on three different Al substrates is shown in Schematic 1 . Previous work has shown that the two-step anodization was beneficial, as the first anodization is crucial for forming a pre-textured Al surface ( Schematic 1 a(ii), b(ii), and c(ii)), which helps grow an organized array of pores during the second anodization step [17, 18]. The pre-textured surface grown during the first anodization step can significantly impact the properties of the resulting anodic oxide film, including pore diameter, density, and distribution [19]. It can also affect the mechanical properties of the anodic oxide film, such as its hardness and wear resistance [20]. The first anodization on a porous surface result in uneven pores, which are removed through chemical etching. A second anodization on the same surface increases the propagation of individual pores through the oxide layer, concentrating the maximum electric field at the pore tip and decreasing towards the pore walls. The continuous anodization process of the Al surface allows pores to grow into the bulk metal, resulting in locally ordered pores with a straight channel through the porous layer, generated from the barrier oxide layer formed at the bottom of the first porous layer [21]. Thus, this study extends a comprehensive comparison by exploring AAO fabrication on both planar and non-planar substrates, such as rod-like wire Al and tubular through-hole Al, presenting a promising avenue to overcoming the challenges associated with the planar AAO configuration [22-26]. Despite extensive research on anodization processes, there is a notable lack of studies focusing on the anodization of both the outer and inner surfaces of Al tubular. While previous studies have explored various anodization techniques, few have addressed these dual surfaces, highlighting a significant gap in current literature. Given this limited focus, this work aims to fill the gap by conducting a rigorous comparative analysis of the anodization process, focusing on the surface morphology influenced by two critical variables of the anodization process: voltage and time. These variables exert a significant influence over the development of AAO on the substrates, impacting attributes such as pore diameter, interpore distance, and pore arrangement. Through this investigation, we aim to deepen our understanding of the intricate relationship between anodization variables and AAO morphology, thereby facilitating informed engineering decisions in various applications. 2.0 Methodology 2.1. Chemicals and Materials The following chemicals and materials were purchased accordingly. High purity (99.99%) Al foil (thickness, 0.1 mm) from Sigma Aldrich Corporation (St. Louis, MO, USA), high purity (99.995%) Al wire (diameter, 1.0 mm) from Alfa Aesar (Tewksbury, MA, USA), and high purity (99.99%) Al hollow tube (diameter, 6.0 mm) from Nilaco Corporation (Ginza, Japan) were used as the anodization substrates for growing AAO. Anhydrous toluene (95%) and acetone (C 3 H 6 O) from Merck KGaA (Darmstadt, Germany). Phosphoric acid (H 3 PO 4 ), 80% oxalic acid (H 2 C 2 O 4 ), chromium (VI) oxide (CrO 3 ), and sodium hydroxide (NaOH) from R&M Chemicals (Selangor, Malaysia). 2.2. Fabrication of AAO on planar and non-planar Al substrates The fabrication of AAO on Al foil involved a two-step anodization process in a custom-built etching cell [1, 27, 28]. The pre-treatment step of the anodized Al planar was conducted by rinsing the Al substrate planar in C 3 H 6 O to eliminate surface contaminants. For the first anodization step, the exposed surface area of the Al planar was immersed in an electrolyte solution containing 0.3 M H 2 C 2 O 4 at a constant temperature of 1 °C for 1 minute under their respective voltages 40, 60, 80, and 100 V. A platinum counter electrode was placed parallel to the Al substrate at a fixed distance (≈ 2 cm) to provide a uniform electric field distribution, in accordance with previously reported studies. Furthermore, continuous stirring of the electrolyte was applied to minimize ion concentration gradients and improve uniform current distribution. After anodization was completed, the resulting anodic layer was stripped off at the end of the first anodization step upon immersion in a mixed solution comprised of 6.0 wt.% H 3 PO 4 and 1.8 wt.% CrO 3 at 80 °C for 1 hour. This treatment left the exposed surface area, which functioned as a template for the second anodization step of the Al planar. The anodization step was then repeated under the same conditions as above, except that the anodization time was extended to 6 hours. The production of the AAO on non-planar surfaces involved two types of Al substrate: Al wire and Al hollow tube and they were cut into 7 - 10 cm lengths. The non-planar Al substrate was then rinsed with C 3 H 6 O and deionized water; a two-step electrochemical anodization process was used in a custom-built anodization cell, as shown in Fig. 1 . The anodization step in non-planar Al substrates is identical to that outlined in planar Al substrates, except for the first anodization time for non-planar, which is increased to 1 hour due to their wall thickness being thicker than the planar substrate. After being treated with an etching solution, the exposed area served as a template for the second anodization step of the Al non-planar. The conditions for the second anodization step closely mirrored those in the planar, as described above. Before morphological characterization, both resulting AAOs were cleaned and dried. 2.3. Morphology characterization The prepared samples were platinum-coated and placed on a conductive carbon tape to examine their morphological properties utilizing a high-resolution field-emission scanning electron microscope (FESEM), Hitachi High-Technologies Corporation in Tokyo, Japan. The analysis was conducted at an accelerating voltage of 5.0 kV with a probe current of 5 nA. The working distance from the electron source to the sample ranges from 4 mm to 8 mm for the best image quality. The FESEM images were captured at various magnifications to achieve comprehensive imaging, specifically at 50,000×. Subsequently, the obtained FESEM images underwent in-depth analysis using ImageJ software (available at https://imagej.nih.gov) by measuring the average of (n = 25) samples. 3.0 Results and Discussion 3.1 AAO on Al planar and non-planar substrates Fig. 2 displays the different original forms of Al substrates before and after the anodization processes. The magnification of the FESEM images contains the inset pore diameter distribution graphs of the Al substrate after the anodization process. Fig. 2 (a-c) depicts all the Al substrates before anodization. Fig. 2 (d) represents the final appearance of AAO films. The use of high-purity Al foil for anodization has resulted in the formation of nanoporous AAO with an almost ideal and hexagonal arrangement of the pores. This result can be supported by a study by Dobosz, who compared the structural features and pore arrangement of AAO in pure and alloy Al foil. The author concluded that the formation of the alumina layer in alloy Al had a worse pore arrangement, and the alloying elements caused severe defects in the porous structure [29]. Fig. 2 (e) shows the ultimate forms of nanoporous AAO wire. The reflective colors of AAO pores can be readily tuned by altering the thickness of the nanoporous AAO. The high applied voltage will cause the thickness of the AAO to increase. This thickness will determine the final color appearance of the AAO. The yellow or gold color of the AAO results when anodizing aluminum using oxalic acid [30, 31]. The resulting oxide layer is pale yellow when anodized at low voltages and can appear more gold-like under high-voltage conditions. Mahmud et al. [22] have reported that using Al wire as a substrate provides strong physical support for nanoporous structures, facilitates flexible and effortless manipulation, and creates a three-dimensional active site with a relatively greater surface area. AAO with a rod-like shape on non-planar substrates presents new opportunities and can be used as an attractive platform for nanomaterials [32, 33], in contrast to AAO on planar substrates, which only offers a one-dimensional active site for functionalization. Fig. 2 (f) shows the finished item of outer AAO on an Al tubular. The hollow tube was 7 cm long, with an effective surface area of 5 to 6 cm. During the anodization, one of the Al ends was not etched and was left in its natural state to facilitate mechanical handling. Fabricating AAO nanopores on both the outer and inner surfaces of an Al hollow tube substrate using electrochemical anodic oxidation is highly appealing. This method provides a greater membrane area per unit volume for AAO growth than planar substrates, as it utilizes both tube surfaces, resulting in a significantly larger surface area for nanopore formation. Given its substantial surface area, AAO in non-planar has significant potential as a catalytic site for chemical reactions, particularly in fields like energy, where applications include advanced gas separation technologies [34, 35]. The high surface area and porosity of the AAO structure provide a substantial contact area between the gas molecules and the porous membrane, which enhances the separation efficiency. The magnification of FESEM images in Fig. 2 shows the top surface of AAO on foil, wire, and hollow tube after the second anodization at 40 V for 6 hours. The nanoporous arrayed structure revealed that the pores are close-packed hexagonal, aligned with a mean pore diameter of 35.0 nm and an interpore distance of approximately 87.0 nm for the Al foil. The average pore diameter of AAO for Al wire and Al hollow tube was measured to be 50.0 nm and 40.0nm, respectively. The interpore distance was approximately 130.0 nm for the wire and 90.0 nm for the outer AAO of the hollow tube. The significant difference in interpore distance between the wire and tube arises from the variation in their exposure geometries. While both substrates possess curved surfaces, the wire has a single continuous surface in contact with the electrolyte. At the same time, the tube presents two opposing surfaces (inner and outer) with different effective exposure areas, and the degree of curvature differs significantly. This phenomenon leads to non-uniform current densities across the tube surfaces, distorting the current distribution and thereby influencing the interpore distance, even under similar anodization voltages. The wire, having a smaller radius of curvature, experiences more substantial local electric field variations compared to the tube. This enhanced field effect influences pore nucleation dynamics and oxide growth, resulting in a larger interpore distance. FESEM images under high magnification show a typical self-ordering regime in the range of 1–2 µm, which can be attributed to the significant volume expansion of Al to alumina under constant voltage [36]. It was also observed from FESEM images that the hexagonal pore cells tend to separate within the grain boundaries, which can be attributed to strong repulsive forces among the pore cells caused by high-volume expansion at high fields [29, 37]. In the fabrication of AAO, pore properties such as pore diameter, interpore distance, and pore thickness are crucial aspects that determine the final structure of AAO [38]. Interpore distance quantifies the average distance between the centers of two consecutive pores, offering valuable information about the material’s porosity. It provides insight into the arrangement and density of the nanopores within the AAO structure. 3.2. Factors influencing the anodization process and AAO structures 3.2.1 Effect of anodization voltage on pore diameters and interpore distance The effect of anodization voltage on the AAO nanoporous structure was investigated while maintaining the electrolyte concentration and temperature constant during anodization. It has been reported that the growth rate, pore diameter, and interpore distance are proportionally increased with the applied voltage during anodization, as incremental changes in the anodization parameter expand the diameter of the nanopore [39-41]. A similar pattern of voltage dependence on pore diameter has also been reported in other works [35, 42]. Fig. 3 summarizes the average pore diameter and interpore distance formed at different anodization voltages for planar and non-planar Al substrates. The pore diameter and interpore distance for all three Al substrates show a steadily increasing pattern at 40, 60, and 80 V; however, when the anodization voltage rises to 100 V, the pore diameter and interpore spacing increase drastically twofold from 80 V. As the anodization voltage applied to planar and non-planar substrates, there is competition between the growth of these individual nanopores and the spacing between them as they grow in the same space. Although the pore diameter and interpore distance positively correlated with anodization voltage in all Al substrates, irregularity of the pore arrangement was noticed in non-planar pore cells. The stress-induced self-ordering mechanism in AAO growth can explain this observation. On planar surfaces, AAO growth tends to be more ordered due to strain equilibration, resulting in smaller and more uniform pore sizes. In contrast, non-planar surfaces experience higher compressive stress, leading to less uniform pore sizes in AAO structures [43, 44]. Even so, pore diameters anodized in planar substrates were lower than those grown on non-planar substrates at the same applied voltage. The interpore distance is a critical parameter that provides insights into the spatial arrangement of pores within a structure. Specifically, a higher interpore distance indicates a lower density of pores per unit of surface area. In the case of AAO, the interpore distance, denoted as D int ,exhibits a linear relationship with the anodization voltage governed by a proportionality constant, approximately z = 2.5 nm/V. Using expression (4) , the D int at voltages 40, 60, 80, and 100 V are computed to be 100, 150, 200, and 250 nm, respectively. D int = zV (4) These findings align with the outcomes of the previous literature [45], reinforcing that voltage plays a pivotal role in modulating the interpore distance within the material. They suggest that the ability to control pore diameter and interpore distance during anodization increases significantly with voltage adjustments throughout the process. The formation of AAO nanopores is also highly dependent on the surface characteristics of the Al substrates. Surface area plays a critical role in determining the density of electric field lines, ion migration rates, and interpore distance [46-48]. A higher surface area provides more nucleation sites for pore formation, potentially leading to a higher pore density. However, interpore distance is primarily influenced by the distribution of the electric field across the surface. The key principle is that the greater the effective surface area per unit volume, the more the electric field is diluted-thereby affecting the final interpore spacing. The interpore distance graph ( Fig. 3 (b)) for all three Al substrates shows that AAO on Al wire exhibits the largest interpore distance, followed by AAO on Al hollow tube, while AAO on Al foil has the smallest. The flat surface of the Al foil has the lowest surface area per unit volume, as it is two-dimensional (2D) and lacks curvature. With the electric field evenly distributed across the surface, the pores grow in a hexagonally ordered arrangement with a regular and relatively small interpore distance. Since there is no curvature to enhance the electric field, no additional spacing between pores occurs. In contrast, the Al wire has a larger surface area per unit volume due to its cylindrical shape. The convex curvature of the wire focuses the electric field at the outermost surface, increasing the electric field intensity. This stronger electric field enhances ion migration across the curved surface. As the oxide grows radially outward, the pores tend to be spaced apart, resulting in a larger interpore distance compared to the flat surface. The Al hollow tube exhibits the highest surface area per unit volume, with both inner (concave) and outer (convex) surfaces exposed to anodization. The outer convex surface behaves similarly to Al wire, where electric field enhancement leads to an increase in interpore distances. However, on the inner concave surface, the electric field lines are more dispersed, reducing field strength and leading to a smaller interpore distance. As a result of these opposing effects, the average interpore distance of the AAO formed on the hollow tube falls between that of the foil and the wire. The interpore distance in AAO is influenced not only by the surface area per unit volume, but the ion transport across the planar and non-planar substrates. During anodization, ion movement behaves differently on planar versus non-planar substrates due to differences in electric field distribution and the effects of curvature. Ion migration primarily involves two species of ions: O²⁻ (oxygen ions) or OH⁻ (hydroxide ions), which migrate from the electrolyte toward the metal-oxide interface, and Al³⁺ (aluminum ions), which migrate from the metal towards the oxide-electrolyte interface. A high anodization voltage provides a strong electric field across the growing oxide and sets the driving force for ion migration. Given by Ohm’s law in ionic conductors, the current density is directly linked to ionic transport. Fig. 4 shows the ion migration pathway in all Al substrates. In the planar Al substrate ( Fig. 4 (a)), which lacks curvature, the electric field is uniformly distributed across the surface. The current-time transients in this substrate show an initial current spike that decays exponentially into a steady-state plateau over time as the barrier layer thickens [49]. As a result, ion migration occurs perpendicular to the surface along straight field lines, leading to vertical pore growth and formation of a highly uniform 2D nanoporous structure. Fig. 4 (b) depicts the convex curvature in the Al wire. The curved surface generates a lateral and radial electric field, which intensifies at the outermost surface. Due to this concentration of the electric field, ion migration follows a slightly outward radial trajectory. This enhances a localized current density at the outer edges and this directional change leads to a larger interpore distance compared to a planar substrate. The current density decay exhibits a similar trend to that of the planar case. Still, a higher initial current density than flat substrate was observed due to the enhanced field concentration in the convex surface of the Al wire. The hollow tube shown in Fig. 4 (c) features both the convex and concave curvatures. In the case of a hollow tube substrate, the field distribution becomes asymmetric. The concave inner wall may experience a higher local electric field due to the converging field concentration, while the outer convex region exhibits comparatively lower electric field and current density. This non-uniform field distribution leads to a more complex transient current behavior and produces distinct pore structures on both the outer and inner sides of the tube. These opposing effects partially counterbalance the electric field enhancement, resulting in a moderate interpore distance compared to that of the Al wire. The FESEM top-view images in Fig. 5 show the nanostructure of AAO for all the Al substrates with variation in anodization voltage. The average pore diameter and the interpore distance of AAO of the Al foil are 36.0 nm and 87.0 nm, respectively, when anodized at 40 V ( Fig. 5 (a)). Those values were smaller than those formed during anodization at 60 V ( Fig. 5 (b)), with pore diameter and interpore distance being 60.0 nm and 120.0 nm, respectively. This observed value for pore diameter and interpore distance is within the range of earlier research [50], yet the obtained value is significantly higher. This might be due to the anodizing temperature. A lower anodizing temperature generally yields smaller pore sizes and closer interpore distances. Higher temperatures increase ionic mobility and enhance Al dissolution rates, resulting in larger, more irregularly shaped pores in the AAO microstructure [51]. Accordingly, the membrane was fabricated at 80 V for 6 hours, as depicted in Fig. 5 (c) shows a larger pore size and interpore distance of 90.3 nm and 190.0 nm, respectively. At 100 V for 6 hours of anodization, as shown in Fig. 5 (d), the pore arrangement becomes less ordered, with an interpore spacing of 250.0 nm and an average pore diameter of 130.0 nm. Similar trends were observed in previous reports where size, pore cells, and arrangements were not uniform when a higher applied voltage was used [52-54]. This can be attributed to the increased current density and high-volume expansion of the pore walls due to the low growth rate of the films. Pore formation is hypothesized to be accompanied by volume expansion during the oxide synthesis at the metal-oxide interface. The oxide is pushed upward due to volume expansion, causing the oxide walls to shift upward and thereby increasing the height of the pore wall. Since higher voltage is associated with higher current densities and higher volume expansion, more oxide is pushed in a tangential and upward direction. More pressure is applied to the pore walls, increasing the pore size, regardless of whether the substrate is planar or non-planar [55]. It is suggested that the dissolution of AAO in the electrolyte follows a \"field-assisted\" mechanism, indicating a strong dependence on voltage potential [56]. It is also worth mentioning that the pore density and porosity of the AAO membrane increase at lower anodization voltages [57, 58]. Fig. 5 (e-h) shows FESEM images of the self-ordered AAO nanopore structure of the Al wire after being treated with two-step anodization for 6 hours. The AAO pore diameter is a function of the anodization voltage that can continuously vary in the 30.0 - 60.0 nm range for AAO anodized at 40 V ( Fig. 5 (e)); anodization potential at 60 V gives AAO pore diameter with a 60.0 - 90.0 nm range ( Fig. 5 (f)), while 90.0 - 120.0 nm for AAO anodized at 80 V ( Fig. 5 (g)) and an impressive 190.0 - 220.0 nm when anodized at 100 V ( Fig. 5 (h)). The interpore distance anodized at 40 V obtained in this work is 139.0 nm, almost double the interpore distance reported in the previous study by Banerjee et al., where their obtained value is around 85.0 nm under the same anodizing conditions [59]. This discrepancy is high due to the Al wire with a larger diameter, which may facilitate a more uniform electric field across the surface during anodization, promoting consistent pore formation and growth [54]. This uniformity can result in larger inter-pore distances, as the pores expand more evenly than those in smaller-diameter wires, where variations may occur. Fig. 5 (i-l) shows the FESEM images of the AAO pattern on the outer side of the Al hollow tube. Fig. 5 (i) reveals a nanopore array structure of outer AAO when anodized at 40 V for 6 hours and the average pore diameter obtained is 41.0 nm. The measured pore diameter of outer AAO on the Al hollow tube in this work was significantly larger than the 30.0 nm pore diameter reported in the previous study under similar anodization conditions [25]. In our study, we utilized high-purity aluminum tubes (99.99%), whereas Belwalkar et al., used alloy tubes (98%) and anodized only the inner surface. These differences in substrate composition and anodization methods likely contribute to the observed variations. As the voltage increased from 40 to 100 V, the pore diameter expanded from 40.0 nm to 205.0 nm, respectively. The FESEM images of all Al substrates in Fig. 5 , strengthened the findings that the higher the applied voltage, the more irregular the AAO pore arrangement, regardless of the substrate used. When the same voltages were applied to these three substrates, AAO in non-planar substrates gave the largest pore structure compared to the pore structure in planar substrates. We further investigated the nanostructure of AAO formed on both the outer and inner surfaces of the Al hollow tube. To our knowledge, no previous studies have reported a detailed comparison of the nanostructure on both sides of the Al substrate. Most literature focuses only on AAO formation on a single side of flat or cylindrical substrates [26, 60]. Fig. 6 shows the FESEM images of the nanostructure of AAO on the outer and inner surfaces of the hollow tube. At an anodization voltage of 40 V, the pore diameter of the outer AAO layer ( Fig. 6 (a)) is larger than that of the inner AAO layer ( Fig. 6 (e)), with average pore diameters of 41.0 nm and 35.3 nm, respectively. This trend remains consistent at higher anodization voltages (60 V to 100 V), where the outer AAO layer exhibits average pore diameters of 83.2 nm, 120.6 nm, and 205.0 nm ( Fig. 6 (b-d)), compared to 74.8 nm, 93.7 nm, and 193.2 nm for the inner AAO layer depicted in Fig. 6 (f-h). A similar pattern is observed for the interpore distance, where the outer AAO layer consistently exhibits larger spacing than the inner AAO layer. This difference can be attributed to the distinct electrochemical environments on the inner and outer surfaces of the tube. The outer surface, which has more direct exposure to the electrolyte, experiences a stronger electric field during anodization [61]. This stronger local electric field enhances pore growth rates and promotes the formation of larger pores. In contrast, the inner surface, partially shielded and confined within the hollow tube, experiences a relatively weaker electric field, resulting in smaller pores and shorter interpore distances. Additionally, the curvature of the hollow tube introduces mechanical stress during anodization, driven by volume expansion during oxide growth and the difference in mechanical constraints between the inner and outer surfaces. This curvature-induced stress, combined with variations in electric field distribution and local electrolyte replenishment, creates a non-uniform anodization environment. Such geometric and mechanical factors contribute to the observed differences in pore size and interpore distance between the inner and outer AAO layers. In addition to the sandwich-like structure observed in the AAO on the Al hollow tube, the formation of nanopores, particularly within the inner AAO layer, offers new insights into the growth dynamics and morphology of anodic alumina on curved surfaces. Fig. 7 illustrates the formation of the inner and outer AAO nanopores on the convex and concave surfaces of the Al hollow tube. The exposure of electrolytes to both sides of the tube allows a double-step anodization process to occur on the inner and outer sides of the tube. The surface morphology of the Al substrates has a high influence on the AAO layer growth. For the pore growth on different surface morphologies, Kopp et al. plotted the normalized thickness of pore growth on a flat surface. On a planar surface, pores grow parallel, resulting in uniform pore lengths throughout the oxide layer. This uniformity in length or thickness also ensures a constant radius at the bottom pore during growth. The normalized value of L/L planar is equal to one and serves as a reference for comparing growth rates across different surface morphologies [62]. On a convex surface, the oxide grows inward toward the Al core, forming a convex base where the electric field becomes concentrated and accelerates dissolution at the pore bottoms. The resulting pores tend to diverge, with the radius of the pore opening being larger than that at the pore bottom. Due to spatial constraints, some pores terminate during the oxide growth process [63]. In contrast, oxide growth on a concave surface produces a different and equally intriguing outcome. In the inner AAO layer facing the inner electrolyte, pores initiate at the inner electrolyte-metal interface and grow outward, away from the Al core, causing the pore bottoms to bulge inward into the metal. On a concave surface, the pores tend to converge at the bottom, meaning the radius of the pores at the bottom is larger than that of the upper pores. Compared to planar surfaces, the difference in pore radius between the upper and bottom layers of both convex and concave geometries leads to variations in electrical field strength throughout the oxide layer. As a result, the oxide growth rate on the curved surface differs from that on planar surfaces. The stronger the substrate curvature, the greater the deviation from the normalized growth rate observed on a planar surface. During AAO formation, whether in concave or convex surfaces, the electric field is strongest at the pore bottoms. This is because the pore bottom is the closest point to the Al-metal interface, where anodic oxidation actively takes place. The curvature at the pore tip concentrates the electric field lines within the confined space, intensifying the field strength [64]. This strong electric field accelerates ionic transport (the inward migration of O²⁻ ions and the outward migration of Al³⁺ ions), thereby accelerating both oxide formation and dissolution at the pore bottom. The electric field at the pore rims-defined as the edges between neighboring pores, is significantly weaker. This is because the rims are located farther from the Al-metal interface and are subjected to a more diffuse electric field. The reduced field intensity slows ionic transport, resulting in slower oxide growth at the rims. The interplay between localized electric fields, oxide dissolution, and oxide formation contributes to the development of the self-organized concave-convex pore structure. Variations in pore curvature can influence fluid flow, adsorption behavior, and surface chemistry of the AAO. 3.2.2. Effect of anodization time on AAO thickness Compared to the anodization’s applied voltage, the treatment time duration has less effect on the pore diameter and interpore spacing [65, 66]. The thickness of the AAO membrane strongly depends on the anodization time, as the formation of pores is directed by the anodization rate. The trend of increasing membrane thickness with longer anodization time has been documented in the literature [67, 68]. This study kept all other anodization conditions constant while varying the anodization time. The cross-sectional FESEM image in Fig. 8 (a) reveals that the overall thickness of the Al foil is approximately 20.0 µm. Higher magnification ( Fig. 8 (b)) images show that the pores grow perpendicular to the surface, extending through the bulk of the thickness with typical non-intercrossing, straight, and smooth cylindrical pores. The cylindrical pore channel is measured to be the same as the pore diameter on the top surface, around 35.0 nm, making them suitable for various nanotechnology applications, including the production of nanomaterials and devices, biological and chemical sensors, nanoelectronics, filter membranes, and tissue engineering scaffolds. Corresponding FESEM images of the AAO layer thickness on Al wire ( Fig. 8 (c,d)) show a more significant effect of anodization time on oxide formation. The anodic oxide layer grown by 40 V anodization voltage for 8 hours in an electrolyte is 18.0 µm thick. By prolonging the duration of the second-step anodization, the pore diameter and interpore distance remained constant, unlike when the applied voltage increased. Regardless of the treatment period, the pore diameter of the nanostructure remained stable, with only the AAO thickness increasing. For the AAO layer on the Al hollow tube, the characteristics formed on this substrate differ from those of the Al wire when creating a double layer of AAO, sandwiching the Al substrates, as depicted in Fig. 8 (e,f). The outer side of the AAO layer under 40 V was 22.5 µm, while the inner side of the AAO layer was 16.0 µm thick. Previous literature suggests that oxide layer formation on curved surfaces behaves differently from that on flat surfaces. Cracks are forming on the surface of the AAO tubular membrane, as illustrated in Fig. 8 (f). The mechanical stress theory explains these phenomena. The internal resistance a material offers to deformation when exposed to external forces is known as stress. When external loads or environmental factors, such as extended anodization times, are applied to a tubular membrane, internal stresses arise within the structure if the stresses exceed the tube's strength limit. Radial cracks do not form in AAO on planar substrates because there is less mechanical tension between the pores. After applying the anodization voltage, the Al converts into alumina. The expanded alumina will produce mechanical stress between the pores, resulting in a hexagonal structure. It has been highlighted by Kasi et al. that tensile stress is higher in tubes with a smaller diameter. Therefore, even though crack formation in non-planar substrates is unavoidable, utilizing larger diameter Al tubes is advised [69]. The rate at which the potential is varied should be gentle enough to create a gradual transition as limiting oxide growth rate is evident with prolonged second anodization. The growth rates reflect the thickness of AAO at various anodization times. Fig. 9 illustrates the effect of anodization time on AAO thickness for both planar and non-planar Al substrates. The oxide layer thickness in this graph is measured from cross-sectional FESEM images of AAO samples prepared using different anodization times. As the anodization time increased from 480 to 1800 minutes, the thickness of the AAO on the Al planar increased from 12.5 ± 2.1 µm to 47.0 ± 1.9 µm. In comparison, the AAO thickness on non-planar Al substrates ranged from 14.0 ± 3.5µm to 60.0 ± 2.7µm. The graph shows a linear relationship between anodization time and AAO thickness, indicating a proportional increase in thickness with longer anodization times. The growth rate of pores on planar surfaces is typically higher than on non-planar surfaces, as previous studies have discussed the effect of curvature on Al foils using lithography techniques [63, 70-72]. In this work, the influence of surface curvature was investigated across three different Al substrates. Experimental observations revealed a growth trend that deviates from the commonly accepted pattern, showing that the pore growth rates vary significantly with curvature. Interestingly, cell proliferation was found to increase with surface curvature, being more prominent on non-planar surfaces than on planar ones. These variations in growth rate are primarily attributed to differences in electric field distribution, ion migration dynamics, and the effects of curvature during anodization. Stronger electric fields at specific curvature regions contributed to accelerated oxide growth. The convex surface in the Al hollow tube enhances the electric field, thereby increasing ion migration and accelerating oxide growth [73, 74]. Meanwhile, the concave surface contributes to additional oxide formation, enabling AAO growth from both the inner and outer surfaces. As a result, the total anodized area is larger than that of other Al substrates, allowing more oxide to form over time [25]. This dual-layer growth leads to the highest measured AAO thickness. The AAO formed on the Al wire exhibits the second-highest thickness. The wire’s convex curvature intensifies the electric field, promoting greater Al 3+ ion migration. This increased ion mobility results in a faster oxide growth rate compared to the flat Al foil. However, unlike the hollow tube, the wire only possesses an outer convex surface, leading to unidirectional oxide growth. Consequently, the AAO layers on the wire are thicker than those on the foil but thinner than those on the hollow tube. In contrast, AAO growth on Al foil shows the lowest pore growth rate. The flat surface creates a uniform electric field, causing ion migration to be evenly distributed without enhancement. Due to the absence of curvature-driven field enhancement, oxide growth occurs at a steady but comparatively slower rate. 4.0 Conclusion The experimental investigation has successfully demonstrated the influence of anodization voltage and time on the nanoporous structure of AAO formed on both planar and non-planar aluminum substrates, including hollow aluminum tubes. Systematic FESEM characterization revealed that increasing anodization voltage led to a proportional increase in pore diameter and interpore distance for both substrate types. Compared to planar substrates, non-planar substrates, particularly aluminum hollow tubes, exhibited larger pore sizes and greater AAO layer thickness at similar anodization voltages, highlighting the impact of substrate geometry on oxide growth dynamics. A key finding of this work is the novel identification and detailed analysis of the dual-layer AAO structure formed on the inner and outer surfaces of the aluminum hollow tube. A larger pore and wider interpore distance were observed on the outer AAO layer that was directly exposed to the bulk electrolyte compared to the inner AAO layer, which faced a confined electrolyte environment. This disparity is attributed to differences in electric field strength, electrolyte replenishment, and mechanical stress between the inner and outer surfaces of the curved tube. These findings provide valuable new insights into the interplay between anodization parameters, substrate geometry, and resulting nanostructure formation. Understanding these relationships is essential for precise engineering of AAO materials, particularly for applications that require tailored pore structures, such as advanced filtration, sensing, and catalysis systems. This work offers a foundation for future research into optimizing AAO structures on complex geometries, further expanding the versatility of AAO across scientific and industrial applications. Declarations 5.1. Funding The authors fully acknowledge Universiti Teknologi MARA and the Ministry of Higher Education (MOHE) Malaysia for the approved funds through the Fundamental grant (FRGS/1 /2022/STG05/UITM/02/2), the UiTM Research Entity Collaboration Grant (KEPU) research funding (600-RMC/KEPU 5/3 (003/2021)) and Vice-Chancellor Special Project (VCSP) Scholarly Nucleus UiTM (600-RMC/VCSP 5/3 (017/2024)). 5.2. Authors’ contribution statement Mawar Hasyikin Abu Seman: Writing, experimental, and analysis data - original draft. Nor Izzati Gati: Experimental and analysis data. Abdul Hadi Mahmud: Experimental and analysis data. Zadariana Jamil: Writing - review & editing. Nafisah Osman: Writing - review & editing. Kim-Fatt Low: Writing - review & editing. Chung-Jen Tseng: Writing - review & editing. Abdul Mutalib Md Jani: Conceptualization, supervision, writing - review & editing. 5.3. 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Supplementary Files Scheme1.png Schematic 1 Formation of AAO pores on (a) Al foil, (b) Al wire, and (c) Al hollow tube through a two-step anodization process Cite Share Download PDF Status: Published Journal Publication published 23 Feb, 2026 Read the published version in Journal of Porous Materials → Version 1 posted Editorial decision: Revision requested 28 Oct, 2025 Reviews received at journal 28 Oct, 2025 Reviews received at journal 27 Oct, 2025 Reviews received at journal 27 Oct, 2025 Reviews received at journal 25 Oct, 2025 Reviews received at journal 16 Oct, 2025 Reviewers agreed at journal 10 Oct, 2025 Reviews received at journal 09 Oct, 2025 Reviewers agreed at journal 09 Oct, 2025 Reviewers agreed at journal 09 Oct, 2025 Reviewers agreed at journal 08 Oct, 2025 Reviewers agreed at journal 08 Oct, 2025 Reviewers agreed at journal 08 Oct, 2025 Reviewers agreed at journal 08 Oct, 2025 Reviewers agreed at journal 07 Oct, 2025 Reviewers agreed at journal 07 Oct, 2025 Reviewers invited by journal 07 Oct, 2025 Editor assigned by journal 06 Oct, 2025 Submission checks completed at journal 06 Oct, 2025 First submitted to journal 06 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Jani\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIiWNgGAWjYDACCQYDECXDJsF8gCEBLJRAjJYEBh42CbYEhoQEUrQwSPAYQFUT0MI/u3nrZt4fdjx80j3fJB7+2MbAz55jwHSzDY8ld46V3eZJSOZhkzm7TSIh4TaDZM8bA+ZcPFoYbuSYAbUwA/2SC9FicCMHvxZ5iJZ6oJacZ2At9oS0GEC0HAZpYYPYIkFAiyHQLzfnpB0H+uWYsUVC2m0eiTPPCg7nnMOtRe5287Ybb2yq5eRnNz+8+cPmthx/e/LGxzlleLyPDnhAxAFGNhK0QMEf0rWMglEwCkbBsAUAwOFPMQ5dE7EAAAAASUVORK5CYII=\",\"orcid\":\"\",\"institution\":\"Universiti Teknologi MARA\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Abdul\",\"middleName\":\"Mutalib Md\",\"lastName\":\"J\",\"suffix\":\"Md\"}],\"badges\":[],\"createdAt\":\"2025-10-06 14:53:15\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-7792391/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-7792391/v1\",\"draftVersion\":[],\"editorialEvents\":[{\"content\":\"https://doi.org/10.1007/s10934-026-01921-2\",\"type\":\"published\",\"date\":\"2026-02-23T15:57:27+00:00\"}],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":93922760,\"identity\":\"1ea7c31f-5d92-4ae0-ad18-153d0911848e\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"docx\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"acdc-reference\",\"size\":3614209,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"ManuscriptJPM2025.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/cd1c047be6e1d93fb45fb0a2.docx\"},{\"id\":93923294,\"identity\":\"bef386d4-dfce-40b3-9586-433fe6f967bd\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 10:04:33\",\"extension\":\"json\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"acdc-reference\",\"size\":9480,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"ca8bf7c4fdfc45fa9d05fe35aca9cec1.json\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/51c61786e3757ad6d7888904.json\"},{\"id\":93922749,\"identity\":\"517ade71-42c0-42fd-9cce-b69e46d5d22d\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":125923,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eSchematic diagram of the experimental setup for the fabrication of AAO\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/bed6838a1d3f963877f5f7eb.png\"},{\"id\":93922754,\"identity\":\"ae994ae7-cf6f-4585-8fc0-79304816022d\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":909868,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003ePhotographs of three Al substrates (a-c) before anodization, and (d-f) after anodization at 40 V in 0.3 M H\\u003csub\\u003e2\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e4\\u003c/sub\\u003e. The magnified top-view FESEM images revealed the nanoporous structure of AAO, and the insets show the graph of their pore diameter distribution\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/655d6e98aeb0eaf2eecbacbe.png\"},{\"id\":93923295,\"identity\":\"dbd2eebe-68ae-4ccf-b02b-2b94445be95f\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 10:04:33\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":366226,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe (a) pore diameter and (b) interpore distance of AAO anodized with three different substrates in 0.3 M H\\u003csub\\u003e2\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e4\\u003c/sub\\u003e with 6 hours of treatment time at various anodizing potentials\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/7056582bc1fbbe5797b568d1.png\"},{\"id\":93922752,\"identity\":\"08b590bf-a4dd-4072-b673-046c785c418b\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":438666,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe mechanism of ion migration at the pore bottom on (a) Al foil, (b) Al wire, and (c) Al hollow tube during the anodization process\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/d9540af53eefdc1b4de3dfd5.png\"},{\"id\":93922756,\"identity\":\"25427c10-e38b-4d25-9f7c-7bb63eb22d04\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":724329,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eFESEM images of AAO on Al substrates: (a-d) foil, (e-h) wire, and (i-l) hollow tube prepared in 0.3 M H\\u003csub\\u003e2\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e4\\u003c/sub\\u003e at anodization time prolonged for 6 hours, and the insets show their pore diameter distribution graph\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"5.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/97f0674476a4fc2f1d22a058.png\"},{\"id\":93923296,\"identity\":\"e36809dc-24b1-4596-b45f-70426e406fc2\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 10:04:33\",\"extension\":\"png\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":1223311,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eComparison of top-view FESEM images of the AAO on (a-d) the outer side and (e-h) the inner side of an Al hollow tube anodized at different anodization voltages for 6 hours\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"6.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/846db8b8f3f0de38304f769e.png\"},{\"id\":93922755,\"identity\":\"a772f074-78c5-4310-9a94-4e724b833b52\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":330840,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe schematic diagram of the nanopores formation on the inner and outer AAO of the Al hollow tube substrates\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"7.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/6a2d07195dc4692d42f847e7.png\"},{\"id\":93922759,\"identity\":\"183c99d8-b517-4b7e-a906-345dcaaf2563\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":8,\"title\":\"Figure 8\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":859826,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eCross-section FESEM images of the AAO layer anodized at 40 V for 8 hours. Low magnification is depicted in (a, c, e) and high magnification of Al foil, Al wire, and Al hollow tube (b, d, f), respectively\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"8.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/08aa890363f64deb1bb20051.png\"},{\"id\":93922758,\"identity\":\"75954945-3ea6-4fee-a5ec-42641ea0ff29\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":9,\"title\":\"Figure 9\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":125284,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eThe graph shows the effect of anodization time on AAO thickness on different Al substrates when anodized at 40 V\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"9.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/6d9c6fd74ff79584de9b3518.png\"},{\"id\":103765704,\"identity\":\"6654e3e6-49f5-43c2-9947-d20471d9dce1\",\"added_by\":\"auto\",\"created_at\":\"2026-03-02 16:07:52\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":6969827,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/13548758-665a-4402-9aed-2ecb62a6d6b6.pdf\"},{\"id\":93922750,\"identity\":\"3fc6837d-0b0a-42c7-bc19-c2971b82eea6\",\"added_by\":\"auto\",\"created_at\":\"2025-10-20 09:56:33\",\"extension\":\"png\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":293504,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eSchematic 1\\u003c/strong\\u003e Formation of AAO pores on (a) Al foil, (b) Al wire, and (c) Al hollow tube through a two-step anodization process\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"Scheme1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-7792391/v1/b311d8d01219a728caa9c9c8.png\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Nanopore Architectures in Anodic Aluminum Oxide: Effects of Anodization Voltage and Time on Planar and Non-Planar Aluminum Substrates\",\"fulltext\":[{\"header\":\"1.0 Introduction\",\"content\":\"\\u003cp\\u003eAnodic aluminum oxide (AAO) refers to a nanoporous structure formed through the electrochemical oxidation process, also known as anodization of aluminum. This process creates a highly ordered arrangement of nanopores, resembling a honeycomb structure. Significant advancements in this field emerged in 1995 with the introduction of a two-step anodization process by Masuda and Fukuda [1]. They proposed a pre-texturing approach to fabricate porous AAO membranes [2]. The anodization process has become an appealing option due to its unique ability to offer precise control over the morphology and thickness of the AAO porous layer. This method has proven to be highly repeatable, offering advantages such as cost-effectiveness, rapid deposition times, ease of operation, and scalability [3]. These attributes make anodization a compelling choice for the production of AAO, enabling researchers to harness its unique properties for a wide range of applications, not only limited as functional nanomaterials to gas separation [4, 5], sensing [6, 7], and energy storage [8, 9], but the usage of the AAO can also be extended as a template for creating nanostructures like nanowires and nanotubes [10, 11].\\u003c/p\\u003e\\n\\u003cp\\u003eThe fabrication of AAO depends on the relationship between anodization conditions and the resulting AAO structure. Precise control over the size and spacing of these nanopores is achieved by adjusting anodization variables. Given that previous studies have extensively explored factors such as electrolyte composition, temperature, and current density [12-15], this study narrows its focus to the impact of anodization voltage and time on AAO formation in planar and non-planar Al substrates. Understanding how these two key parameters influence pore structure and oxide growth can further optimize AAO fabrication for various applications.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u0026nbsp;In particular, the growth process of porous AAO involves a balance between the reactions that occur at both the anode and cathode. The primary response at the anode consists of the oxidation of aluminum to form aluminum oxide (Al\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e3\\u003c/sub\\u003e). The general electrochemical anodizing processes can be represented as equations below:\\u003c/p\\u003e\\n\\u003ctable border=\\\"0\\\" cellspacing=\\\"0\\\" cellpadding=\\\"0\\\" width=\\\"614\\\"\\u003e\\n \\u003ctbody\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd valign=\\\"top\\\" style=\\\"width: 35.342%;\\\"\\u003e\\n \\u003cp\\u003eOxidation of Al at the anode:\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 44.6254%;\\\"\\u003e\\n \\u003cp\\u003e2Al + 3H\\u003csub\\u003e2\\u003c/sub\\u003eO\\u0026nbsp;\\u0026reg;\\u0026nbsp;Al\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e3\\u003c/sub\\u003e + 6H\\u003csup\\u003e+\\u003c/sup\\u003e + 6e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd valign=\\\"top\\\" style=\\\"width: 20.0326%;\\\"\\u003e\\n \\u003cp\\u003e\\u003cstrong\\u003e(1)\\u003c/strong\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd valign=\\\"top\\\" style=\\\"width: 35.342%;\\\"\\u003e\\n \\u003cp\\u003eReduction of H\\u003csup\\u003e+\\u003c/sup\\u003e at the cathode:\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 44.6254%;\\\"\\u003e\\n \\u003cp\\u003e6H\\u003csup\\u003e+\\u0026nbsp;\\u003c/sup\\u003e+ 6e\\u0026nbsp;\\u0026reg;\\u0026nbsp;3H\\u003csub\\u003e2\\u003c/sub\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd valign=\\\"top\\\" style=\\\"width: 20.0326%;\\\"\\u003e\\n \\u003cp\\u003e\\u003cstrong\\u003e(2)\\u003c/strong\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd valign=\\\"top\\\" style=\\\"width: 35.342%;\\\"\\u003e\\n \\u003cp\\u003eOverall reaction:\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 44.6254%;\\\"\\u003e\\n \\u003cp\\u003e2Al + 3H\\u003csub\\u003e2\\u003c/sub\\u003eO\\u0026nbsp;\\u0026reg;\\u0026nbsp;Al\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e3\\u0026nbsp;\\u003c/sub\\u003e+ 3H\\u003csub\\u003e2\\u003c/sub\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd valign=\\\"top\\\" style=\\\"width: 20.0326%;\\\"\\u003e\\n \\u003cp\\u003e\\u003cstrong\\u003e(3)\\u003c/strong\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003c/tbody\\u003e\\n\\u003c/table\\u003e\\n\\u003cp\\u003eThe most remarkable feature of AAO lies in its nanoporous structures, characterized by highly ordered pores or channels that form a quasi-hexagonal, close-packed arrangement. AAO can be engineered with specific nanostructures, encompassing pore size, interpore distance, pore density, and oxide layer thickness. Typically, AAO is configured as a bulky membrane attached to an Al foil substrate, maintaining a flat profile [16]. However, conventional planar AAO membranes have inherent limitations, particularly their brittleness and difficulties in handling. Moreover, they provide only a one-dimensional surface for functionalization. It makes non-planar AAO structures, such as wires and tubes, an attractive alternative to explore. While adopting non-planar configurations does not eliminate brittleness, the presence of the underlying metallic substrate provides additional mechanical support, which helps to alleviate some of the handling challenges. Additionally, non-planar AAO offers multidimensional surfaces for functionalization, thereby expanding the range of potential applications.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe proposed schematic formation of AAO on three different Al substrates is shown in \\u003cstrong\\u003eSchematic 1\\u003c/strong\\u003e. Previous work has shown that the two-step anodization was beneficial, as the first anodization is crucial for forming a pre-textured Al surface (\\u003cstrong\\u003eSchematic 1\\u003c/strong\\u003ea(ii), b(ii), and c(ii)), which helps grow an organized array of pores during the second anodization step [17, 18]. The pre-textured surface grown during the first anodization step can significantly impact the properties of the resulting anodic oxide film, including pore diameter, density, and distribution [19]. It can also affect the mechanical properties of the anodic oxide film, such as its hardness and wear resistance [20].\\u003c/p\\u003e\\n\\u003cp\\u003eThe first anodization on a porous surface result in uneven pores, which are removed through chemical etching. A second anodization on the same surface increases the propagation of individual pores through the oxide layer, concentrating the maximum electric field at the pore tip and decreasing towards the pore walls. The continuous anodization process of the Al surface allows pores to grow into the bulk metal, resulting in locally ordered pores with a straight channel through the porous layer, generated from the barrier oxide layer formed at the bottom of the first porous layer [21].\\u003c/p\\u003e\\n\\u003cp\\u003eThus, this study extends a comprehensive comparison by exploring AAO fabrication on both planar and non-planar substrates, such as rod-like wire Al and tubular through-hole Al, presenting a promising avenue to overcoming the challenges associated with the planar AAO configuration [22-26]. Despite extensive research on anodization processes, there is a notable lack of studies focusing on the anodization of both the outer and inner surfaces of Al tubular. While previous studies have explored various anodization techniques, few have addressed these dual surfaces, highlighting a significant gap in current literature. Given this limited focus, this work aims to fill the gap by conducting a rigorous comparative analysis of the anodization process, focusing on the surface morphology influenced by two critical variables of the anodization process: voltage and time. These variables exert a significant influence over the development of AAO on the substrates, impacting attributes such as pore diameter, interpore distance, and pore arrangement. Through this investigation, we aim to deepen our understanding of the intricate relationship between anodization variables and AAO morphology, thereby facilitating informed engineering decisions in various applications.\\u003c/p\\u003e\"},{\"header\":\"2.0 Methodology\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003e2.1.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;Chemicals and Materials\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe following chemicals and materials were purchased accordingly. High purity (99.99%) Al foil (thickness, 0.1 mm) from Sigma Aldrich Corporation (St. Louis, MO, USA), high purity (99.995%) Al wire (diameter, 1.0 mm) from Alfa Aesar (Tewksbury, MA, USA), and high purity (99.99%) Al hollow tube (diameter, 6.0 mm) from Nilaco Corporation (Ginza, Japan) were used as the anodization substrates for growing AAO. Anhydrous toluene (95%) and acetone (C\\u003csub\\u003e3\\u003c/sub\\u003eH\\u003csub\\u003e6\\u003c/sub\\u003eO) from Merck KGaA (Darmstadt, Germany). Phosphoric acid (H\\u003csub\\u003e3\\u003c/sub\\u003ePO\\u003csub\\u003e4\\u003c/sub\\u003e), 80% oxalic acid (H\\u003csub\\u003e2\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e4\\u003c/sub\\u003e), chromium (VI) oxide (CrO\\u003csub\\u003e3\\u003c/sub\\u003e), and sodium hydroxide (NaOH) from R\\u0026amp;M Chemicals (Selangor, Malaysia).\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e2.2.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;Fabrication of AAO on planar and non-planar Al substrates\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe fabrication of AAO on Al foil involved a two-step anodization process in a custom-built etching cell [1, 27, 28]. The pre-treatment step of the anodized Al planar was conducted by rinsing the Al substrate planar in C\\u003csub\\u003e3\\u003c/sub\\u003eH\\u003csub\\u003e6\\u003c/sub\\u003eO to eliminate surface contaminants. For the first anodization step, the exposed surface area of the Al planar was immersed in an electrolyte solution containing 0.3 M H\\u003csub\\u003e2\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eO\\u003csub\\u003e4\\u003c/sub\\u003e at a constant temperature of 1 °C for 1 minute under their respective voltages 40, 60, 80, and 100 V.\\u0026nbsp;A platinum counter electrode was placed parallel to the Al substrate at a fixed distance (≈ 2 cm) to provide a uniform electric field distribution, in accordance with previously reported studies. Furthermore, continuous stirring of the electrolyte was applied to minimize ion concentration gradients and improve uniform current distribution.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eAfter anodization was completed, the resulting anodic layer was stripped off at the end of the first anodization step upon immersion in a mixed solution comprised of 6.0 wt.% H\\u003csub\\u003e3\\u003c/sub\\u003ePO\\u003csub\\u003e4\\u003c/sub\\u003e and 1.8 wt.% CrO\\u003csub\\u003e3\\u0026nbsp;\\u003c/sub\\u003eat 80 °C for 1 hour. \\u0026nbsp;This treatment left the exposed surface area, which functioned as a template for the second anodization step of the Al planar. The anodization step was then repeated under the same conditions as above, except that the anodization time was extended to 6 hours. The production of the AAO on non-planar surfaces involved two types of Al substrate: Al wire and Al hollow tube and they were cut into 7 - 10 cm lengths. The non-planar Al substrate was then rinsed with C\\u003csub\\u003e3\\u003c/sub\\u003eH\\u003csub\\u003e6\\u003c/sub\\u003eO and deionized water; a two-step electrochemical anodization process was used in a custom-built anodization cell, as shown in \\u003cstrong\\u003eFig. 1\\u003c/strong\\u003e. The anodization step in non-planar Al substrates is identical to that outlined in planar Al substrates, except for the first anodization time for non-planar, which is increased to 1 hour due to their wall thickness being thicker than the planar substrate. After being treated with an etching solution, the exposed area served as a template for the second anodization step of the Al non-planar. The conditions for the second anodization step closely mirrored those in the planar, as described above. Before morphological characterization, both resulting AAOs were cleaned and dried.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e2.3.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;Morphology characterization\\u0026nbsp;\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe prepared samples were platinum-coated and placed on a conductive carbon tape to examine their morphological properties utilizing a high-resolution field-emission scanning electron microscope (FESEM), Hitachi High-Technologies Corporation in Tokyo, Japan. The analysis was conducted at an accelerating voltage of 5.0 kV with a probe current of 5 nA. The working distance from the electron source to the sample ranges from 4 mm to 8 mm for the best image quality. The FESEM images were captured at various magnifications to achieve comprehensive imaging, specifically at 50,000×. Subsequently, the obtained FESEM images underwent in-depth analysis using ImageJ software (available at https://imagej.nih.gov) by measuring the average of (n = 25) samples.\\u003c/p\\u003e\"},{\"header\":\"3.0 Results and Discussion\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003e3.1\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;AAO on Al planar and non-planar substrates\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFig. 2\\u003c/strong\\u003e displays the different original forms of Al substrates before and after the anodization processes. The magnification of the FESEM images contains the inset pore diameter distribution graphs of the Al substrate after the anodization process. \\u003cstrong\\u003eFig. 2\\u003c/strong\\u003e(a-c) depicts all the Al substrates before anodization. \\u003cstrong\\u003eFig. 2\\u003c/strong\\u003e(d) represents the final appearance of AAO films. The use of high-purity Al foil for anodization has resulted in the formation of nanoporous AAO with an almost ideal and hexagonal arrangement of the pores. This result can be supported by a study by Dobosz, who compared the structural features and pore arrangement of AAO in pure and alloy Al foil. The author concluded that the formation of the alumina layer in alloy Al had a worse pore arrangement, and the alloying elements caused severe defects in the porous structure [29].\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFig. 2\\u003c/strong\\u003e(e) shows the ultimate forms of nanoporous AAO wire. The reflective colors of AAO pores can be readily tuned by altering the thickness of the nanoporous AAO. The high applied voltage will cause the thickness of the AAO to increase. This thickness will determine the final color appearance of the AAO. The yellow or gold color of the AAO results when anodizing aluminum using oxalic acid [30, 31]. The resulting oxide layer is pale yellow when anodized at low voltages and can appear more gold-like under high-voltage conditions. Mahmud et al. [22] have reported that using Al wire as a substrate provides strong physical support for nanoporous structures, facilitates flexible and effortless manipulation, and creates a three-dimensional active site with a relatively greater surface area. AAO with a rod-like shape on non-planar substrates presents new opportunities and can be used as an attractive platform for nanomaterials [32, 33], in contrast to AAO on planar substrates, which only offers a one-dimensional active site for functionalization.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFig. 2\\u003c/strong\\u003e(f) shows the finished item of outer AAO on an Al tubular. The hollow tube was 7 cm long, with an effective surface area of 5 to 6 cm. During the anodization, one of the Al ends was not etched and was left in its natural state to facilitate mechanical handling. Fabricating AAO nanopores on both the outer and inner surfaces of an Al hollow tube substrate using electrochemical anodic oxidation is highly appealing. This method provides a greater membrane area per unit volume for AAO growth than planar substrates, as it utilizes both tube surfaces, resulting in a significantly larger surface area for nanopore formation. Given its substantial surface area, AAO in non-planar has significant potential as a catalytic site for chemical reactions, particularly in fields like energy, where applications include advanced gas separation technologies [34, 35]. The high surface area and porosity of the AAO structure provide a substantial contact area between the gas molecules and the porous membrane, which enhances the separation efficiency.\\u003c/p\\u003e\\n\\u003cp\\u003eThe magnification of FESEM images in \\u003cstrong\\u003eFig. 2\\u003c/strong\\u003e shows the top surface of AAO on foil, wire, and hollow tube after the second anodization at 40 V for 6 hours. The nanoporous arrayed structure revealed that the pores are close-packed hexagonal, aligned with a mean pore diameter of 35.0 nm and an interpore distance of approximately 87.0 nm for the Al foil. The average pore diameter of AAO for Al wire and Al hollow tube was measured to be 50.0 nm and 40.0nm, respectively. The interpore distance was approximately 130.0 nm for the wire and 90.0 nm for the outer AAO of the hollow tube. The significant difference in interpore distance between the wire and tube arises from the variation in their exposure geometries. While both substrates possess curved surfaces, the wire has a single continuous surface in contact with the electrolyte. At the same time, the tube presents two opposing surfaces (inner and outer) with different effective exposure areas, and the degree of curvature differs significantly. This phenomenon leads to non-uniform current densities across the tube surfaces, distorting the current distribution and thereby influencing the interpore distance, even under similar anodization voltages. The wire, having a smaller radius of curvature, experiences more substantial local electric field variations compared to the tube. This enhanced field effect influences pore nucleation dynamics and oxide growth, resulting in a larger interpore distance. FESEM images under high magnification show a typical self-ordering regime in the range of 1–2 µm, which can be attributed to the significant volume expansion of Al to alumina under constant voltage [36]. It was also observed from FESEM images that the hexagonal pore cells tend to separate within the grain boundaries, which can be attributed to strong repulsive forces among the pore cells caused by high-volume expansion at high fields [29, 37]. In the fabrication of AAO, pore properties such as pore diameter, interpore distance, and pore thickness are crucial aspects that determine the final structure of AAO [38]. Interpore distance quantifies the average distance between the centers of two consecutive pores, offering valuable information about the material’s porosity. It provides insight into the arrangement and density of the nanopores within the AAO structure.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e3.2.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;Factors influencing the anodization process and AAO structures\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e3.2.1\\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;Effect of anodization voltage on pore diameters and interpore distance\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe effect of anodization voltage on the AAO nanoporous structure was investigated while maintaining the electrolyte concentration and temperature constant during anodization. It has been reported that the growth rate, pore diameter, and interpore distance are proportionally increased with the applied voltage during anodization, as incremental changes in the anodization parameter expand the diameter of the nanopore [39-41]. A similar pattern of voltage dependence on pore diameter has also been reported in other works [35, 42]. \\u003cstrong\\u003eFig. 3\\u003c/strong\\u003e summarizes the average pore diameter and interpore distance formed at different anodization voltages for planar and non-planar Al substrates. The pore diameter and interpore distance for all three Al substrates show a steadily increasing pattern at 40, 60, and 80 V; however, when the anodization voltage rises to 100 V, the pore diameter and interpore spacing increase drastically twofold from 80 V. As the anodization voltage applied to planar and non-planar substrates, there is competition between the growth of these individual nanopores and the spacing between them as they grow in the same space. Although the pore diameter and interpore distance positively correlated with anodization voltage in all Al substrates, irregularity of the pore arrangement was noticed in non-planar pore cells. The stress-induced self-ordering mechanism in AAO growth can explain this observation. On planar surfaces, AAO growth tends to be more ordered due to strain equilibration, resulting in smaller and more uniform pore sizes. In contrast, non-planar surfaces experience higher compressive stress, leading to less uniform pore sizes in AAO structures [43, 44]. Even so, pore diameters anodized in planar substrates were lower than those grown on non-planar substrates at the same applied voltage.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe interpore distance is a critical parameter that provides insights into the spatial arrangement of pores within a structure. Specifically, a higher interpore distance indicates a lower density of pores per unit of surface area. In the case of AAO, the interpore distance, denoted as \\u003cem\\u003eD\\u003c/em\\u003e\\u003csub\\u003eint\\u003c/sub\\u003e,exhibits a linear relationship with the anodization voltage governed by a proportionality constant, approximately\\u0026nbsp;z\\u0026nbsp;= 2.5 nm/V. Using expression \\u003cstrong\\u003e(4)\\u003c/strong\\u003e, the \\u003cem\\u003eD\\u003c/em\\u003e\\u003csub\\u003eint\\u003c/sub\\u003e at voltages 40, 60, 80, and 100 V are computed to be 100, 150, 200, and 250 nm, respectively.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003ctable border=\\\"0\\\" cellspacing=\\\"0\\\" cellpadding=\\\"0\\\"\\u003e\\n \\u003ctbody\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd valign=\\\"top\\\"\\u003e\\n \\u003cp\\u003e\\u003cem\\u003eD\\u003c/em\\u003e\\u003csub\\u003eint =\\u0026nbsp;\\u003c/sub\\u003ezV\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd valign=\\\"top\\\"\\u003e\\n \\u003cp\\u003e\\u003cstrong\\u003e(4)\\u003c/strong\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003c/tbody\\u003e\\n\\u003c/table\\u003e\\n\\u003cp\\u003eThese findings align with the outcomes of the previous literature [45], reinforcing that voltage plays a pivotal role in modulating the interpore distance within the material. They suggest that the ability to control pore diameter and interpore distance during anodization increases significantly with voltage adjustments throughout the process. The formation of AAO nanopores is also highly dependent on the surface characteristics of the Al substrates. Surface area plays a critical role in determining the density of electric field lines, ion migration rates, and interpore distance [46-48]. A higher surface area provides more nucleation sites for pore formation, potentially leading to a higher pore density. However, interpore distance is primarily influenced by the distribution of the electric field across the surface. The key principle is that the greater the effective surface area per unit volume, the more the electric field is diluted-thereby affecting the final interpore spacing.\\u003c/p\\u003e\\n\\u003cp\\u003eThe interpore distance graph (\\u003cstrong\\u003eFig. 3\\u003c/strong\\u003e(b)) for all three Al substrates shows that AAO on Al wire exhibits the largest interpore distance, followed by AAO on Al hollow tube, while AAO on Al foil has the smallest. The flat surface of the Al foil has the lowest surface area per unit volume, as it is two-dimensional (2D) and lacks curvature. With the electric field evenly distributed across the surface, the pores grow in a hexagonally ordered arrangement with a regular and relatively small interpore distance. Since there is no curvature to enhance the electric field, no additional spacing between pores occurs.\\u003c/p\\u003e\\n\\u003cp\\u003eIn contrast, the Al wire has a larger surface area per unit volume due to its cylindrical shape. The convex curvature of the wire focuses the electric field at the outermost surface, increasing the electric field intensity. This stronger electric field enhances ion migration across the curved surface. As the oxide grows radially outward, the pores tend to be spaced apart, resulting in a larger interpore distance compared to the flat surface. The Al hollow tube exhibits the highest surface area per unit volume, with both inner (concave) and outer (convex) surfaces exposed to anodization. The outer convex surface behaves similarly to Al wire, where electric field enhancement leads to an increase in interpore distances. However, on the inner concave surface, the electric field lines are more dispersed, reducing field strength and leading to a smaller interpore distance. As a result of these opposing effects, the average interpore distance of the AAO formed on the hollow tube falls between that of the foil and the wire.\\u003c/p\\u003e\\n\\u003cp\\u003eThe interpore distance in AAO is influenced not only by the surface area per unit volume, but the ion transport across the planar and non-planar substrates. During anodization, ion movement behaves differently on planar versus non-planar substrates\\u0026nbsp;due to differences in\\u0026nbsp;electric field distribution and the effects of curvature. Ion migration primarily involves two species of ions: O²⁻ (oxygen ions) or OH⁻ (hydroxide ions), which migrate from the electrolyte toward the metal-oxide interface, and Al³⁺ (aluminum ions), which\\u0026nbsp;migrate from the metal towards the oxide-electrolyte interface.\\u003c/p\\u003e\\n\\u003cp\\u003eA high anodization voltage provides a strong electric field across the growing oxide and sets the driving force for ion migration. Given by Ohm’s law in ionic conductors, the current density is directly linked to ionic transport. \\u003cstrong\\u003eFig. 4\\u003c/strong\\u003e shows the ion migration pathway in all Al substrates. In the planar Al substrate (\\u003cstrong\\u003eFig. 4\\u003c/strong\\u003e(a)), which lacks curvature, the electric field is uniformly distributed across the surface. The current-time transients in this substrate show an initial current spike that decays exponentially into a steady-state plateau over time as the barrier layer thickens [49]. As a result, ion migration occurs perpendicular to the surface along straight field lines, leading to vertical pore growth and formation of a highly uniform 2D nanoporous structure. \\u003cstrong\\u003eFig. 4\\u003c/strong\\u003e(b) depicts the convex curvature in the Al wire. The curved surface generates a lateral and radial electric field, which intensifies at the outermost surface. Due to this concentration of the electric field, ion migration follows a slightly outward radial trajectory. This enhances a localized current density at the outer edges and this directional change leads to a larger interpore distance compared to a planar substrate. The current density decay exhibits a similar trend to that of the planar case. Still, a higher initial current density than flat substrate was observed due to the enhanced field concentration in the convex surface of the Al wire.\\u003c/p\\u003e\\n\\u003cp\\u003eThe hollow tube shown in \\u003cstrong\\u003eFig. 4\\u003c/strong\\u003e(c) features both the convex and concave curvatures. In the case of a hollow tube substrate, the field distribution becomes asymmetric. The concave inner wall may experience a higher local electric field due to the converging field concentration, while the outer convex region exhibits comparatively lower electric field and current density. This non-uniform field distribution leads to a more complex transient current behavior and produces distinct pore structures on both the outer and inner sides of the tube. These opposing effects partially counterbalance the electric field enhancement, resulting in a moderate interpore distance compared to that of the Al wire.\\u003c/p\\u003e\\n\\u003cp\\u003eThe FESEM top-view images in \\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e show the nanostructure of AAO for all the Al substrates with variation in anodization voltage. The average pore diameter and the interpore distance of AAO of the Al foil are 36.0 nm and 87.0 nm, respectively, when anodized at 40 V (\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(a)). Those values were smaller than those formed during anodization at 60 V (\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(b)), with pore diameter and interpore distance being 60.0 nm and\\u0026nbsp;120.0 nm,\\u0026nbsp;respectively. This observed value for pore diameter and interpore distance is within the range of earlier research [50], yet the obtained value is significantly higher. This might be due to the anodizing temperature. A lower anodizing temperature generally yields smaller pore sizes and closer interpore distances. Higher temperatures increase ionic mobility and enhance Al dissolution rates, resulting in larger, more irregularly shaped pores in the AAO microstructure [51].\\u003c/p\\u003e\\n\\u003cp\\u003eAccordingly, the membrane was fabricated at 80 V for 6 hours, as depicted in \\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(c) shows a larger pore size and interpore distance of 90.3 nm and 190.0 nm, respectively. At 100 V for 6 hours of anodization, as shown in \\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(d), the pore arrangement becomes less ordered, with an interpore spacing of 250.0 nm and an average pore diameter of 130.0 nm. Similar trends were observed in previous reports where size, pore cells, and arrangements were not uniform when a higher applied voltage was used [52-54]. This can be attributed to the increased current density and high-volume expansion of the pore walls due to the low growth rate of the films.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003ePore formation is hypothesized to be accompanied by volume expansion during the oxide synthesis at the metal-oxide interface. The oxide is pushed upward due to volume expansion, causing the oxide walls to shift upward and thereby increasing the height of the pore wall. Since higher voltage is associated with higher current densities and higher volume expansion, more oxide is pushed in a tangential and upward direction. More pressure is applied to the pore walls, increasing the pore size, regardless of whether the substrate is planar or non-planar [55]. It is suggested that the dissolution of AAO in the electrolyte follows a \\\"field-assisted\\\" mechanism, indicating a strong dependence on voltage potential [56]. It is also worth mentioning that the pore density and porosity of the AAO membrane increase at lower anodization voltages [57, 58].\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(e-h) shows FESEM images of the self-ordered AAO nanopore structure of the Al wire after being treated with two-step anodization for 6 hours. The AAO pore diameter is a function of the anodization voltage that can continuously vary in the 30.0 - 60.0 nm range for AAO anodized at 40 V (\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(e)); anodization potential at 60 V gives AAO pore diameter with a 60.0 - 90.0 nm range (\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(f)), while 90.0 - 120.0 nm for AAO anodized at 80 V (\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(g)) and an impressive 190.0 - 220.0 nm when anodized at 100 V (\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(h)). The interpore distance anodized at 40 V obtained in this work is 139.0 nm, almost double the interpore distance reported in the previous study by Banerjee et al., where their obtained value is around 85.0 nm under the same anodizing conditions [59]. This discrepancy is high due to the Al wire with a larger diameter, which may facilitate a more uniform electric field across the surface during anodization, promoting consistent pore formation and growth [54]. This uniformity can result in larger inter-pore distances, as the pores expand more evenly than those in smaller-diameter wires, where variations may occur.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(i-l) shows the FESEM images of the AAO pattern on the outer side of the Al hollow tube. \\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e(i) reveals a nanopore array structure of outer AAO when anodized at 40 V for 6 hours and the average pore diameter obtained is 41.0 nm. The measured pore diameter of outer AAO on the Al hollow tube in this work was significantly larger than the 30.0 nm pore diameter reported in the previous study under similar anodization conditions [25]. \\u0026nbsp;In our study, we utilized high-purity aluminum tubes (99.99%), whereas Belwalkar et al., used alloy tubes (98%) and anodized only the inner surface. These differences in substrate composition and anodization methods likely contribute to the observed variations. As the voltage increased from 40 to 100 V, the pore diameter expanded from 40.0 nm to 205.0 nm, respectively. The FESEM images of all Al substrates in \\u003cstrong\\u003eFig. 5\\u003c/strong\\u003e, strengthened the findings that the higher the applied voltage, the more irregular the AAO pore arrangement, regardless of the substrate used. When the same voltages were applied to these three substrates, AAO in non-planar substrates gave the largest pore structure compared to the pore structure in planar substrates.\\u003c/p\\u003e\\n\\u003cp\\u003eWe further investigated the nanostructure of AAO formed on both the outer and inner surfaces of the Al hollow tube. To our knowledge, no previous studies have reported a detailed comparison of the nanostructure on both sides of the Al substrate. Most literature focuses only on AAO formation on a single side of flat or cylindrical substrates [26, 60]. \\u003cstrong\\u003eFig. 6\\u003c/strong\\u003e shows the FESEM images of the nanostructure of AAO on the outer and inner surfaces of the hollow tube. At an anodization voltage of 40 V, the pore diameter of the outer AAO layer (\\u003cstrong\\u003eFig. 6\\u003c/strong\\u003e(a)) is larger than that of the inner AAO layer (\\u003cstrong\\u003eFig. 6\\u003c/strong\\u003e(e)), with average pore diameters of 41.0 nm and 35.3 nm, respectively. This trend remains consistent at higher anodization voltages (60 V to 100 V), where the outer AAO layer exhibits average pore diameters of 83.2 nm, 120.6 nm, and 205.0 nm (\\u003cstrong\\u003eFig. 6\\u003c/strong\\u003e(b-d)), compared to 74.8 nm, 93.7 nm, and 193.2 nm for the inner AAO layer depicted in \\u003cstrong\\u003eFig. 6\\u003c/strong\\u003e(f-h). A similar pattern is observed for the interpore distance, where the outer AAO layer consistently exhibits larger spacing than the inner AAO layer.\\u003c/p\\u003e\\n\\u003cp\\u003eThis difference can be attributed to the distinct electrochemical environments on the inner and outer surfaces of the tube. The outer surface, which has more direct exposure to the electrolyte, experiences a stronger electric field during anodization [61]. This stronger local electric field enhances pore growth rates and promotes the formation of larger pores. In contrast, the inner surface, partially shielded and confined within the hollow tube, experiences a relatively weaker electric field, resulting in smaller pores and shorter interpore distances. Additionally, the curvature of the hollow tube introduces mechanical stress during anodization, driven by volume expansion during oxide growth and the difference in mechanical constraints between the inner and outer surfaces. This curvature-induced stress, combined with variations in electric field distribution and local electrolyte replenishment, creates a non-uniform anodization environment. Such geometric and mechanical factors contribute to the observed differences in pore size and interpore distance between the inner and outer AAO layers.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u0026nbsp;In addition to the sandwich-like structure observed in the AAO on the Al hollow tube, the formation of nanopores, particularly within the inner AAO layer, offers new insights into the growth dynamics and morphology of anodic alumina on curved surfaces. \\u003cstrong\\u003eFig. 7\\u003c/strong\\u003e illustrates the formation of the inner and outer AAO nanopores on the convex and concave surfaces of the Al hollow tube. The exposure of electrolytes to both sides of the tube allows a double-step anodization process to occur on the inner and outer sides of the tube. The surface morphology of the Al substrates has a high influence on the AAO layer growth. For the pore growth on different surface morphologies, Kopp et al. plotted the normalized thickness of pore growth on a flat surface. On a planar surface, pores grow parallel, resulting in uniform pore lengths throughout the oxide layer. This uniformity in length or thickness also ensures a constant radius at the bottom pore during growth. The normalized value of L/L\\u003csub\\u003eplanar\\u003c/sub\\u003e is equal to one and serves as a reference for comparing growth rates across different surface morphologies [62].\\u003c/p\\u003e\\n\\u003cp\\u003eOn a convex surface, the oxide grows inward toward the Al core, forming a convex base where the electric field becomes concentrated and accelerates dissolution at the pore bottoms. The resulting pores tend to diverge, with the radius of the pore opening being larger than that at the pore bottom. Due to spatial constraints, some pores terminate during the oxide growth process [63]. In contrast, oxide growth on a concave surface produces a different and equally intriguing outcome. In the inner AAO layer facing the inner electrolyte, pores initiate at the inner electrolyte-metal interface and grow outward, away from the Al core, causing the pore bottoms to bulge inward into the metal. On a concave surface, the pores tend to converge at the bottom, meaning the radius of the pores at the bottom is larger than that of the upper pores. Compared to planar surfaces, the difference in pore radius between the upper and bottom layers of both convex and concave geometries leads to variations in electrical field strength throughout the oxide layer. As a result, the oxide growth rate on the curved surface differs from that on planar surfaces. The stronger the substrate curvature, the greater the deviation from the normalized growth rate observed on a planar surface.\\u003c/p\\u003e\\n\\u003cp\\u003eDuring AAO formation, whether in concave or convex surfaces, the electric field is strongest at the pore bottoms. This is because the pore bottom is the closest point to the Al-metal interface, where anodic oxidation actively takes place. The curvature at the pore tip concentrates the electric field lines within the confined space, intensifying the field strength [64]. This strong electric field accelerates ionic transport (the inward migration of O²⁻ ions and the outward migration of Al³⁺ ions), thereby accelerating both oxide formation and dissolution at the pore bottom.\\u003c/p\\u003e\\n\\u003cp\\u003eThe electric field at the pore rims-defined as the edges between neighboring pores, is significantly weaker. This is because the rims are located farther from the Al-metal interface and are subjected to a more diffuse electric field. The reduced field intensity slows ionic transport, resulting in slower oxide growth at the rims. The interplay between localized electric fields, oxide dissolution, and oxide formation contributes to the development of the self-organized concave-convex pore structure. Variations in pore curvature can influence fluid flow, adsorption behavior, and surface chemistry of the AAO.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e3.2.2.\\u0026nbsp; \\u0026nbsp; \\u0026nbsp;Effect of anodization time on AAO thickness\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eCompared to the anodization’s applied voltage, the treatment time duration has less effect on the pore diameter and interpore spacing [65, 66]. The thickness of the AAO membrane strongly depends on the anodization time, as the formation of pores is directed by the anodization rate. The trend of increasing membrane thickness with longer anodization time has been documented in the literature [67, 68]. This study kept all other anodization conditions constant while varying the anodization time. The cross-sectional FESEM image in \\u003cstrong\\u003eFig. 8\\u003c/strong\\u003e(a) reveals that the overall thickness of the Al foil is approximately 20.0 µm. Higher magnification (\\u003cstrong\\u003eFig. 8\\u003c/strong\\u003e(b)) images show that the pores grow perpendicular to the surface, extending through the bulk of the thickness with typical non-intercrossing, straight, and smooth cylindrical pores. The cylindrical pore channel is measured to be the same as the pore diameter on the top surface, around 35.0 nm, making them suitable for various nanotechnology applications, including the production of nanomaterials and devices, biological and chemical sensors, nanoelectronics, filter membranes, and tissue engineering scaffolds.\\u003c/p\\u003e\\n\\u003cp\\u003eCorresponding FESEM images of the AAO layer thickness on Al wire (\\u003cstrong\\u003eFig. 8\\u003c/strong\\u003e(c,d)) show a more significant effect of anodization time on oxide formation. The anodic oxide layer grown by 40 V anodization voltage for 8 hours in an electrolyte is 18.0 µm thick. By prolonging the duration of the second-step anodization, the pore diameter and interpore distance remained constant, unlike when the applied voltage increased. Regardless of the treatment period, the pore diameter of the nanostructure remained stable, with only the AAO thickness increasing. For the AAO layer on the Al hollow tube, the characteristics formed on this substrate differ from those of the Al wire when creating a double layer of AAO, sandwiching the Al substrates, as depicted in \\u003cstrong\\u003eFig. 8\\u003c/strong\\u003e(e,f). The outer side of the AAO layer under 40 V was 22.5 µm, while the inner side of the AAO layer was 16.0 µm thick. Previous literature suggests that oxide layer formation on curved surfaces behaves differently from that on flat surfaces. Cracks are forming on the surface of the AAO tubular membrane, as illustrated in \\u003cstrong\\u003eFig. 8\\u003c/strong\\u003e(f).\\u003c/p\\u003e\\n\\u003cp\\u003eThe mechanical stress theory explains these phenomena. The internal resistance a material offers to deformation when exposed to external forces is known as stress. When external loads or environmental factors, such as extended anodization times, are applied to a tubular membrane, internal stresses arise within the structure if the stresses exceed the tube's strength limit. Radial cracks do not form in AAO on planar substrates because there is less mechanical tension between the pores. After applying the anodization voltage, the Al converts into alumina. The expanded alumina will produce mechanical stress between the pores, resulting in a hexagonal structure. It has been highlighted by Kasi et al. that tensile stress is higher in tubes with a smaller diameter. Therefore, even though crack formation in non-planar substrates is unavoidable, utilizing larger diameter Al tubes is advised [69]. The rate at which the potential is varied should be gentle enough to create a gradual transition as limiting oxide growth rate is evident with prolonged second anodization.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe growth rates reflect the thickness of AAO at various anodization times. \\u003cstrong\\u003eFig. 9\\u003c/strong\\u003e illustrates the effect of anodization time on AAO thickness for both planar and non-planar Al substrates. The oxide layer thickness in this graph is measured from cross-sectional FESEM images of AAO samples prepared using different anodization times. As the anodization time increased from 480 to 1800 minutes, the thickness of the AAO on the Al planar increased from 12.5 ± 2.1 µm to 47.0 ± 1.9 µm. In comparison, the AAO thickness on non-planar Al substrates ranged from 14.0 ± 3.5µm to 60.0 ± 2.7µm. The graph shows a linear relationship between anodization time and AAO thickness, indicating a proportional increase in thickness with longer anodization times.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe growth rate of pores on planar surfaces is typically higher than on non-planar surfaces, as previous studies have discussed the effect of curvature on Al foils using lithography techniques [63, 70-72]. In this work, the influence of surface curvature was investigated across three different Al substrates. Experimental observations revealed a growth trend that deviates from the commonly accepted pattern, showing that the pore growth rates vary significantly with curvature. Interestingly, cell proliferation was found to increase with surface curvature, being more prominent on non-planar surfaces than on planar ones. These variations in growth rate are primarily attributed to differences in electric field distribution, ion migration dynamics, and the effects of curvature during anodization. Stronger electric fields at specific curvature regions contributed to accelerated oxide growth.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eThe convex surface in the Al hollow tube enhances the electric field, thereby increasing ion migration and accelerating oxide growth [73, 74]. Meanwhile, the concave surface contributes to additional oxide formation, enabling AAO growth from both the inner and outer surfaces. As a result, the total anodized area is larger than that of other Al substrates, allowing more oxide to form over time [25]. This dual-layer growth leads to the highest measured AAO thickness. The AAO formed on the Al wire exhibits the second-highest thickness. The wire’s convex curvature intensifies the electric field, promoting greater Al\\u003csup\\u003e3+\\u0026nbsp;\\u003c/sup\\u003eion migration. This increased ion mobility results in a faster oxide growth rate compared to the flat Al foil. However, unlike the hollow tube, the wire only possesses an outer convex surface, leading to unidirectional oxide growth. Consequently, the AAO layers on the wire are thicker than those on the foil but thinner than those on the hollow tube. In contrast, AAO growth on Al foil shows the lowest pore growth rate. The flat surface creates a uniform electric field, causing ion migration to be evenly distributed without enhancement. Due to the absence of curvature-driven field enhancement, oxide growth occurs at a steady but comparatively slower rate.\\u0026nbsp;\\u003c/p\\u003e\"},{\"header\":\"4.0 Conclusion\",\"content\":\"\\u003cp\\u003eThe experimental investigation has successfully demonstrated the influence of anodization voltage and time on the nanoporous structure of AAO formed on both planar and non-planar aluminum substrates, including hollow aluminum tubes. Systematic FESEM characterization revealed that increasing anodization voltage led to a proportional increase in pore diameter and interpore distance for both substrate types. Compared to planar substrates, non-planar substrates, particularly aluminum hollow tubes, exhibited larger pore sizes and greater AAO layer thickness at similar anodization voltages, highlighting the impact of substrate geometry on oxide growth dynamics. A key finding of this work is the novel identification and detailed analysis of the dual-layer AAO structure formed on the inner and outer surfaces of the aluminum hollow tube. A larger pore and wider interpore distance were observed on the outer AAO layer that was directly exposed to the bulk electrolyte compared to the inner AAO layer, which faced a confined electrolyte environment. This disparity is attributed to differences in electric field strength, electrolyte replenishment, and mechanical stress between the inner and outer surfaces of the curved tube. These findings provide valuable new insights into the interplay between anodization parameters, substrate geometry, and resulting nanostructure formation. Understanding these relationships is essential for precise engineering of AAO materials, particularly for applications that require tailored pore structures, such as advanced filtration, sensing, and catalysis systems. This work offers a foundation for future research into optimizing AAO structures on complex geometries, further expanding the versatility of AAO across scientific and industrial applications.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003e5.1.\\u0026nbsp; \\u0026nbsp;Funding\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors fully acknowledge Universiti Teknologi MARA and the Ministry of Higher Education (MOHE) Malaysia for the approved funds through the Fundamental grant (FRGS/1 /2022/STG05/UITM/02/2), the UiTM Research Entity Collaboration Grant (KEPU) research funding (600-RMC/KEPU 5/3 (003/2021)) and\\u0026nbsp;Vice-Chancellor Special Project (VCSP) Scholarly Nucleus UiTM (600-RMC/VCSP 5/3 (017/2024)).\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e5.2.\\u0026nbsp; \\u0026nbsp;Authors’ contribution statement\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eMawar Hasyikin Abu Seman:\\u003c/strong\\u003e Writing, experimental, and analysis data - original draft. \\u003cstrong\\u003eNor Izzati Gati:\\u0026nbsp;\\u003c/strong\\u003eExperimental and analysis data. \\u003cstrong\\u003eAbdul Hadi Mahmud:\\u003c/strong\\u003e Experimental and analysis data. \\u003cstrong\\u003eZadariana Jamil:\\u003c/strong\\u003e Writing - review \\u0026amp; editing. \\u003cstrong\\u003eNafisah Osman:\\u003c/strong\\u003e Writing - review \\u0026amp; editing. \\u003cstrong\\u003eKim-Fatt Low:\\u003c/strong\\u003e Writing - review \\u0026amp; editing. \\u003cstrong\\u003eChung-Jen Tseng:\\u003c/strong\\u003e Writing - review \\u0026amp; editing. \\u003cstrong\\u003eAbdul Mutalib Md Jani:\\u003c/strong\\u003e Conceptualization, supervision, writing - review \\u0026amp; editing.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003e5.3.\\u0026nbsp; \\u0026nbsp;Competing interest\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors have no competing interests to declare that are relevant to the content of this article.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eH. 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Jaskuła, Anodic alumina membranes with defined pore diameters and thicknesses obtained by adjusting the anodizing duration and pore opening/widening time, Journal of Solid State Electrochemistry, 15 (2011) 2427-2436.\\u003c/li\\u003e\\n\\u003cli\\u003eD. Elabar, T. Hashimoto, J. Qi, P. Skeldon, G.E. Thompson, Effect of low levels of sulphate on the current density and film morphology during anodizing of aluminium in chromic acid, Electrochimica Acta, 196 (2016) 206-222.\\u003c/li\\u003e\\n\\u003cli\\u003eH.N. Chamidy, A. Ngatin, A.F. Rosyadi, A. Julviana, N. Noviyani, Effect of Voltage on the Thickness of Oxide Layer at Aluminum Alloys for Structural Bonding Using Phosphoric Sulfuric Acid Anodizing (Psa) Process, International Journal of Mechanical Engineering Technologies and Applications, 4 (2023) 69-76.\\u003c/li\\u003e\\n\\u003cli\\u003eJ.K. Kasi, A.K. Kasi, M. Bokhari, M. Sohail, Characterization of cracks in tubular anodic aluminum oxide membrane, Am. J. Condens. Matter Phys, 6 (2016) 36-40.\\u003c/li\\u003e\\n\\u003cli\\u003eA. Yin, R.S. Guico, J. Xu, Fabrication of anodic aluminium oxide templates on curved surfaces, Nanotechnology, 18 (2007) 035304.\\u003c/li\\u003e\\n\\u003cli\\u003eC.-K. Chung, M.-W. Liao, C.-T. Lee, H.-C. Chang, Anodization of nanoporous alumina on impurity-induced hemisphere curved surface of aluminum at room temperature, Nanoscale research letters, 6 (2011) 1-6.\\u003c/li\\u003e\\n\\u003cli\\u003eM. Schneider, W. F\\u0026uuml;rbeth, Anodizing\\u0026mdash;The pore makes the difference, Materials and Corrosion, 73 (2022) 1752-1765.\\u003c/li\\u003e\\n\\u003cli\\u003eJ. Oh, C.V. Thompson, The role of electric field in pore formation during aluminum anodization, Electrochimica Acta, 56 (2011) 4044-4051.\\u003c/li\\u003e\\n\\u003cli\\u003eJ.M. Runge, J.M. Runge, Anodic aluminum oxide growth and structure, The Metallurgy of Anodizing Aluminum: Connecting Science to Practice, (2018) 281-320.\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"},{\"header\":\"Scheme 1\",\"content\":\"\\u003cp\\u003eScheme 1 is available in the Supplementary Files section.\\u003c/p\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"journal-of-porous-materials\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"jopo\",\"sideBox\":\"Learn more about [Journal of Porous Materials](http://link.springer.com/journal/10934)\",\"snPcode\":\"10934\",\"submissionUrl\":\"https://submission.nature.com/new-submission/10934/3\",\"title\":\"Journal of Porous Materials\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"em\",\"reportingPortfolio\":\"Springer Hybrid\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false},\"keywords\":\"AAO, Al wire, Al hollow tube, AAO nanostructure, two-step anodization\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-7792391/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-7792391/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"This research investigates the effect of voltage changes and anodization duration on the nanopore architecture of anodic aluminum oxide (AAO) across various aluminum (Al) substrates, including both planar and non-planar forms. After a two-step anodization, planar Al substrates developed a porous layer limited to their flat surface. In contrast, the Al wire, due to its curved shape, experienced an intensified local electric field, resulting in thicker oxide layers around its circumference. The most significant effect was observed in hollow Al tubes, where nanoporous layers formed simultaneously on both the inner and outer surfaces. This dual-surface anodization significantly increased the effective surface area and produced the thickest oxide layers among all tested substrates. Field-emission scanning electron microscopy was used to analyze the morphology of the AAO. The findings indicated a direct relationship between the applied voltage and the diameter of AAO pores, with pore sizes increasing from 30.0 to 150.0 nm for planar substrates and from 30.0 to 220.0 nm for non-planar substrates as the voltage increased from 40 V to 100 V. AAO thickness ranged from 12.7 to 47.0 µm for planar substrates and from 14.0 to 60.0 µm for non-planar substrates. The surface structure of the Al substrates also influenced the distribution of AAO pore diameters. The dual-layer AAO on Al tubes exhibited larger pores and greater interpore distances, which can be attributed to differences in oxide growth direction and electrochemical field distribution. These findings offer valuable guidance for the engineering of non-planar AAO materials for diverse applications.\",\"manuscriptTitle\":\"Nanopore Architectures in Anodic Aluminum Oxide: Effects of Anodization Voltage and Time on Planar and Non-Planar Aluminum Substrates\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-10-20 09:56:29\",\"doi\":\"10.21203/rs.3.rs-7792391/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0},{\"type\":\"decision\",\"content\":\"Revision 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