Understanding and Shaping the Morphology of the Barrier Layer of

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Understanding and Shaping the Morphology of the Barrier Layer of Supported Porous Anodized Alumina on Gold Underlayers Nele Berger and Mohammed Es-Souni* Institute for Materials and Surface Technology, University of Applied Sciences Kiel, Grenzstrasse 3, 24149 Kiel, Germany ABSTRACT: Large-area ordered nanorod (NR) arrays of various functional materials can be easily and cost-effectively processed using on-substrate anodized porous aluminum oxide (PAO) films as templates. However, reproducibility in the processing of PAO films is still an issue because they are prone to delamination, and control of fabrication parameters such as electrolyte type and concentration and anodizing time is critical for making robust templates and subsequently mechanically reliable NR arrays. In the present work, we systematically investigate the effects of the fabrication parameters on pore base morphology, devise a method to avoid delamination, and control void formation under the barrier layer of PAO films on gold underlayers. Via systematic control of the anodization parameters, particularly the anodization current density and time, we follow the different stages of void development and discuss their formation mechanisms. The practical aspect of this work demonstrates how void size can be controlled and how void formation can be utilized to control the shape of NR bases for improving the mechanical stability of the NRs.

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INTRODUCTION NR arrays of noble metals and other functional materials are anisotropic nanostructures with peculiar optical, magnetic, and other functional properties that make them particularly suitable for different applications, such as energy storage, nanobiosensing, molecular detection, catalysis, and gas sensing.1−10 Large areas of robust, homogeneous, and long-range ordered NR arrays can be easily and cost-effectively fabricated using supported PAO templates that are obtained via anodizing a thin film of aluminum deposited on a substrate.8,11−18 The PAO films usually have an ordered pore structure which makes them suitable for the fabrication of on-substrate ordered nanostructures.19−21 Usually, one starts with a thin film heterostructure comprising Ti-adhesion layers and a gold layer that serves as a back-contact for the anodization of the Al top layer. An overview of the applied fabrication process is shown in Figure 1. However, the processing of mechanically reliable PAO films is still a challenge, and issues related to pore base morphology, e.g., bottleneck morphology, may entail poor mechanical stability of the NRs (see below). In previous work, it has been reported that anodization of thin aluminum films on gold underlayers or silicon entailed void formation between the underlayer and the barrier layer of the PAO film (the remaining oxide layer that separates PAO from the nonanodized metal), which results in an inverted hemispherical barrier layer that generally leads to delamination of the anodized film.22−24 Crouse et al. explained void formation on silicon by dissolution of the alumina due to local temperature increase or electricfield-enhanced dissolution near the interface.24 Seo et al. believed it was due to the stress distribution caused by volume expansion related to oxide formation.25 Liu et al. examined the formation of voids between PAO films and ITO substrates with © 2016 American Chemical Society

Ti interlayers of different thickness and came to the conclusion that the voids were caused by oxygen bubbles.26 However, none of the previous authors were able to control void formation to achieve uniform NR bases with controllable shapes. Oh and Thompson reported on a method of avoiding these voids by using tungsten underlayers instead of gold. According to them, W is oxidized during anodization and forms penetrating oxide plugs that after selective etching result in broad pore bases penetrating the underlying W layer, and thus prevent delamination of the PAO film.22 Nonetheless, a gold underlayer forms unequaled ohmic contact and is best suited for growing noble metal nanostructures, as it provides perfect nucleation and growth sites. The question then arises as to how to maintain PAO film integrity while keeping the gold underlayer with all its advantages. In this work, we show that proper choice of the anodization conditions allows controlling void formation and morphology of the barrier layer on gold underlayers without impairing mechanical stability of the PAO films. We further show that morphology control of the barrier layer can positively impact the resulting NRs. By controlling the size of the voids, one can mold different shapes of NR bases, which can be useful for improving the mechanical stability of NR arrays. We examine the correlation between the nature of the electrolyte used for anodization, its concentration, and the anodization time on the shape of the barrier layer and discuss underlying mechanisms. Received: May 7, 2016 Revised: June 15, 2016 Published: June 17, 2016 6985

DOI: 10.1021/acs.langmuir.6b01732 Langmuir 2016, 32, 6985−6990

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Figure 1. Fabrication of PAO films starts with coating a silicon wafer with a metallic heterostructure of Ti (6 nm)/Au (15 nm)/Ti (2 nm) via sputtering followed by a layer of 500 nm of Al deposited via e-beam evaporation using high deposition rates to limit grain growth. The coated substrates are then anodized to obtain PAO films. The barrier layer is removed in H3PO4, which also widens the pores. Deposition of noble metals, e.g., platinum, into the pores can be done via electrochemical deposition. After removing the PAO template film using NaOH, an array of freestanding Pt NRs is obtained.

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RESULTS AND DISCUSSION Figure 2 shows the evolution of anodization current density vs time obtained when an aluminum film is anodized in an acid

Figure 3. Cross sections of platinum-NR arrays obtained from deposition into templates fabricated with (a) 0.3 M sulfuric acid and (b) 0.6 M oxalic acid. For both samples, the anodization process was interrupted at point 4 on the current density vs time curve shown in Figure 2. Notice the bottleneck morphology (arrows) of the NR bases in part a. Anodization in oxalic acid leads to an almost cylindrical morphology of the bases, but the bottleneck morphology can still be observed. NRs in part c are fabricated with 0.6 M oxalic acid, but the anodization time is slightly extended (to point 6 in Figure 2), which leads to a slightly enlarged diameter and larger NR bases. NRs in part d are fabricated with 0.2 M oxalic acid. The process was interrupted at the same stage of anodization as in part c. The increase of the NR diameter and the size of the NR base can clearly be seen.

Figure 2. Anodization current density vs time exemplary shown for anodization in 0.2 M oxalic acid. The anodization curves are similar for other electrolytes and electrolyte concentrations. The vertical lines show the different stages of anodization described in the main text. The process was interrupted at points 1, 2, 4, 5, and 6.

electrolyte. Usually, the anodization process is interrupted when the current density starts to rise, i.e., at point 4 in Figure 2. Depending on the electrolyte type and its concentration, one may obtain different morphologies of the pore bases, after barrier layer dissolution and pore opening. To illustrate barrier layer effects on nanostructure morphology, Pt NR arrays were fabricated using templates obtained with 0.3 M sulfuric acid and 0.6 M oxalic acid, respectively (Figure 3). For both samples, anodization of the templates was interrupted at point 4 in Figure 2, as mentioned before. The NRs in both samples have bottleneck-like bases that understandably are less stable mechanically with a high risk of mechanical collapse. One

notices, however, that the diameter of the bases is larger for the sample produced using oxalic acid. The bases in both sample types can be slightly enlarged each by extending the anodization time. Comparing two samples fabricated using the same electrolyte with different concentrations and stopped at the same stage of the anodization process, one can observe 6986

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Langmuir differences in the NR diameters and in the diameters of NR bases (Figure 3c and d). From the results above, it appears that both electrolyte type and its concentration affect the morphology of the pore base and, consequently, that of the grown nanostructures. The question then arises as to how the anodization time in its turn would affect the pore base morphology. Using a low concentration of oxalic acid results in tapered bases, whose size differs depending on the anodization time, as shown Figure 4, which depicts NRs grown in templates fabricated with the

Figure 5. Cross section of PAO film anodized using 0.3 M oxalic acid at 40 V, (a) before pore widening, voids are visible at the bottom of each pore and the barrier layer has a hemispherical shape (arrows); (b) the same sample after widening the pores using 5 wt % phosphoric acid at 30 °C for 27 min, the barrier layer is penetrated leaving an edge (arrows); (c) the edge can be reduced by extending the opening time to 28 min. A complete removal of any remains of the barrier layer is possible, if the interpore distance (from center to center) is wide enough.

drift through the oxide layer path toward the anode due to the high electric field. As long as there is metallic Al, these ions react with Al to form Al2O3. The released H+ ions drift through the oxide layer in the opposite direction and dissolve Al2O3. This process has been described in the literature.27,28 Since the highest H+ concentration occurs at the pore bottoms, the dissolution proceeds faster in the axial than in the radial direction, which leads to vertical pore growth. Figure 6a shows

Figure 4. Cross section of a platinum-NR array obtained from deposition into a template fabricated with 0.2 M oxalic acid. For the sample shown in part a, the anodization process was interrupted between points 4 and 5 (see Figure 2), and for the sample shown in part b, it was interrupted at point 6. This illustrates the influence of the anodization time on the size and shape of NR bases.

same electrolyte concentration but different anodization time. The bases of the NRs shown in Figure 4a are almost cylindrical, while the bases shown in Figure 4b appear wide and cone shaped. These results unambiguously demonstrate that the shape of pore bases depends on the type of electrolyte, its concentration, and the anodization time. The specific shapes of the nanostructure bases depicted above replicate the voids under the barrier layer of each pore. Upon examination of the cross section of different templates, one can observe a difference in the size of the void under the barrier layer of each pore (see below). When the barrier layer is removed by dissolving PAO in phosphoric acid, the whole surface of the AAO reacts with the acid with the etching direction being vertical to the surface. Therefore, the pores are also widened during this step. The voids conserve the same width because the barrier layer shields them from the dissolving acid until the barrier layer is completely removed. This leads to different base shapes. Since the thickness of the pore walls is larger than the thickness of the barrier layer, the removal of the barrier layer occurs before the side walls are completely dissolved. The temperature and time of the etching process have to be accurate enough to achieve a complete removal of the barrier layer without destroying the pores. Figure 5 shows an anodized film before (a) and after (b) pore widening. At the bottom of the widened pores, one can see the remains of the dissolved hemispherical barrier layer. If the distance between neighboring pores is wide enough, these can be removed by further extending the pore opening time (c). In order to systematically trace pore base morphology changes during the anodization process and gain insight into the operating mechanisms underlying barrier layer morphology, we have conducted anodization experiments using one electrolyte concentration and interrupting the anodization process at different sets of current density and time, depicted in Figure 2. During the anodization process, O2− and OH− ions

Figure 6. (a) Schematic diagram showing pore, barrier-layer formation and the participating ions in the reactions. (b) SEM picture of the cross section of an anodized film with anodization being interrupted during pore growth (point 1 in Figure 2). The arrows in part b show the remaining Al film and the barrier layer.

a schematic of this stage of the process, and in Figure 6b, one can see the corresponding SEM cross section picture of the anodized film when the process is interrupted at point 1 in Figure 2. When the pores have reached the adhesion layer, the template becomes transparent and the Ti layer and the remaining Al between the pores are oxidized. The beginning of this step is also characterized by a decrease of the current density (point 2 in Figure 2). Schematics of this phase and the corresponding SEM picture are shown in Figure 7. The formation of oxygen bubbles starts when the metals (at the exception of the Au underlayer) are completely oxidized. At this stadium, O2− and OH− ions still drift toward the anode. However, since there is no metal left to be oxidized, oxygen concentrates at the Al2O3/Au interface. Additionally, water electrolysis transforms water into O2 and H3O+:

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4OH− → O2 + 2H 2O + 4e−

(1)

6H 2O → O2 + 4H3O+ + 4e−

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Al2O3 continues; therefore, the barrier layer is not only lifted off but also thinned. A schematic and the corresponding SEM picture are shown in Figure 9.

Figure 9. (a) Schematic diagram and (b) SEM cross section of an anodized film; anodization was interrupted during void formation (point 5 in Figure 2). At this point, the oxygen bubble pressure is sufficient to lift off the barrier layer, resulting in pronounced void formation (arrows).

As the expansion of the voids continues, after a certain time, the anodized layer is separated from the gold layer, creating a porous Al2O3 membrane. The situation just before delamination can be seen in Figure 10. The delamination is characterized by a sudden drastic increase of current density and can occur suddenly before the film is delaminated completely, point 6 in Figure 2.

Figure 7. Schematic diagrams showing (a) the pores reaching the Ti layer (point 2 in Figure 2) and (b) oxidation of the remaining Al and the Ti layer (point 3 in Figure 2). (c) SEM picture of the cross section of anodized film; anodization was interrupted at point 2 in Figure 2, i.e., as the current began to decrease.

H3O+ ions drift toward the cathode, while O2 forms bubbles between the gold layer and the Al2O3 layer. The phase of bubble formation is characterized by an increase of the current density (point 4 in Figure 2). These bubbles merge and start to form voids. A schematic of this stage is shown in Figure 8.

Figure 10. (a) Schematic diagram and (b) SEM cross section of an anodized film; anodization was interrupted at the beginning of delamination characterized by a drastic increase of current density (point 6 in Figure 2). At this point, delamination of the PAO film starts as indicated by arrows.

Figure 8. (a) Schematic diagram and (b) SEM picture of the cross section of anodized film; anodization was interrupted at the time when the current density began to increase (point 4 in Figure 2). At this point, Al and Ti are completely oxidized and O2 bubble evolution starts with the beginning of void formation (arrows).

If the underlayer consists of a metal that can be oxidized under the same conditions as Al, OH− and O2− ions oxidize this metal instead of producing accumulations of oxygen, which leads to the formation of metal oxide plugs described by Oh and Thompson.22 The ongoing dissolution of Al2O3 leads to removal of the barrier layer above those plugs, whereas using a gold underlayer always leads to the formation of voids. The size of the voids, and thus the NR bases, can be tuned by varying the anodization time between the minimum time, which is characterized by the drop of current density during the anodization process, and the maximum time right before the delamination of the template. For templates with larger pores and interpore distances, which are produced by using lower electrolyte concentrations at higher voltages, the process is easier to control, because wider gaps between the pores allow producing larger voids without delamination.

Due to unevenness and little deviations in pore growth, some pores can start showing voids while others are still in the oxidation state. To obtain a preferably uniform result, the roughness of the layers must be as small as possible. We achieved a relatively even surface by using high deposition rates for Al deposition. When the bubbles accumulate, their pressure eventually attains a critical value sufficient to lift off the oxide layer at the weakest points of the layer, which is in the middle of each pore. This phenomenon leads to the formation of the above-mentioned hemispherical voids. Since the accumulation of oxygen continues as long as the electric field is present, the voids grow larger when the anodization time is extended. This leads to the above-mentioned enlargement of the nanorod bases (see Figure 3). During this process, the dissolution of 6988

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CONCLUSION In conclusion, a proper control of the anodization parameters, particularly the anodization time and current density at a given electrolyte concentration, allows shaping the morphology of PAO films, even when Au underlayers are used as electrodes. We follow the different stages of void formation as a function of current density and rationalize the formation mechanisms in terms of operating electrochemical reactions. It is suggested that void formation is primarily due to oxygen bubble formation between the barrier layer and the Au underlayer after complete oxidation of Al and Ti. Controlling current density during anodization, and pore opening time after anodization allows one to shape pore base morphology. This in turns allows controlling the size and shape of NR bases of template-fabricated NRs. The design options range from thin, bottleneck-like bases to straight bases all the way to large cone shaped bases, which helps in achieving higher mechanical stability of NR arrays for applications in (electro)catalysis, energy storage, and sensing.

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*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Table 1. Combinations of Electrolyte, Concentration, Anodization Temperature, and Pore Opening Time at 30 °C in H3PO4 anodization temperature, °C

opening time, min

sulfuric acid oxalic acid oxalic acid oxalic acid

0.3 0.2 0.3 0.6

5 4 5 23

22 44 28 24

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EXPERIMENTAL SECTION

concentration, mol/L

AUTHOR INFORMATION

Corresponding Author

The following chemicals were used as purchased: oxalic acid dehydrate 99% (Roth, Germany), phosphoric acid 88% (Roth, Germany), sulfuric acid 95−98% (Roth, Germany), hydrogen hexachloroplatinate(IV) solution (Fluka, Germany), and sodium hydroxide (Roth, Germany). Deionized water was used to prepare aqueous solutions. The deposition of the thin film Ti(6 nm)/Au(15 nm)/Ti(2 nm)/Al(500 nm) heterostructure on silicon substrates was carried out by DC-magnetron sputtering and E-beam evaporation using a PVD device (PVD75, Lesker). E-beam evaporation (for Al) was performed at high deposition rates of approximately 10 Å/s to restrict grain growth and obtain a low surface roughness. Anodization was conducted under potentiostatic conditions using 0.3 M sulfuric acid at a voltage of 18.5 V and oxalic acid with concentrations of 0.2, 0.3, and 0.6 M at anodization voltages of 70, 40, and 30 V, respectively. The electrolytes (except the one with 0.6 M, to avoid phase separation) were cooled, which slows down the process, leads to a more uniform pore growth, and makes it easier to control. For the anodization using 0.2 M oxalic acid, the process was interrupted at different stages of anodization, as shown in Figure 2. An electrochemical workstation (Keithley 2400 SM) served to carry out the anodization. A Pt foil was used as a counter electrode. After anodization, the barrier layer was removed using 5 wt % phosphoric acid at 30 °C. Combinations of electrolyte, concentration, anodization temperature, and pore opening time at 30 °C in H3PO4 are shown in Table 1. For an examination of the resulting nanostructure, platinum

electrolyte

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was deposited into the templates via electrochemical deposition using an electrochemical workstation (Princeton Potentiostat/Galvanostat Model 263A) with a Pt foil and Ag/AgCl (saturated KCl) as counter and reference electrodes, respectively. The templates were removed by immersion of the samples in 0.5 wt % NaOH to expose the nanostructures. The microstructure and morphology of the templates and NR arrays were examined with a high resolution scanning electron microscope (Ultra Plus, Zeiss, Germany). 6989

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DOI: 10.1021/acs.langmuir.6b01732 Langmuir 2016, 32, 6985−6990