Name: simultaneous multi-surface anodizations and stair-like reverse biases detachment of anodic aluminum oxides in sulfuric and oxalic acid electrolyte
Uploaded: 2017-10-05T14:00:00.000+00:00
Duration: 10 min 27 s
Description: Watch this Scientific Journal Video about Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte at JoVE.com

The authors have nothing to disclose.

Please be aware of all the related materials safety data sheets (MSDS) before beginning. In spite of the eco-friendly nature of this protocol, a few acids and oxidizers are used in the corresponding procedures. Also, use all the proper personal protective equipment (lab coat, gloves, safety glasses, etc.).
1. Preparation of Solution
Note: After complete sealing of the solution-containing vessel, vigorous magnetic stirring was applied to all the solutions at room temperature in sufficient time.
<ol>
	<li>Preparation of perchloric acid solution
	<ol>
		<li>Mix 100 mL of perchloric acid (HClO4, 60%) with 400 mL of absolute ethanol (C2H5OH, 100%) in 1 to 4 volume ratio.</li>
	</ol>
	</li>
	<li>Preparation of chromic acid solution
	<ol>
		<li>Dissolve 9.0 g chromic oxide (CrO3) and 20.3 mL phosphoric acid (H3PO4, 85%) in 479.7 mL deionized (D.I.) water (CrO3:H3PO4 = 0.18 M:0.56 M).</li>
	</ol>
	</li>
	<li>Preparation of sulfuric acid electrolyte
	<ol>
		<li>Mix 16.2 mL sulfuric acid (H2SO4, 98%) in 983.8 mL D.I. water resulting in molar concentration of 0.3 M.</li>
	</ol>
	</li>
	<li>Preparation of oxalic acid electrolyte
	<ol>
		<li>Dissolve 27.012 g anhydrous oxalic acid (C2H2O4) in 1 L D.I. water resulting in molar concentration of 0.3 M.</li>
	</ol>
	</li>
</ol>
2. Pretreatment of Al Substrate
<ol>
	<li>Machining Al specimen
	<ol>
		<li>Cut the purified Al specimen (&#62;99.99% pure) into rectangular parallelepiped shape (width x height x thickness = 20.0 mm x 50.0 mm x 1.0 mm) with right angles between all the adjacent surfaces, which is referred as &#34;substrate&#34; hereafter.</li>
		<li>Polish the multi-surfaces of the Al substrate mechanically using sandpaper with proper ISO/FEPA Grit Designation number more than P1000. 
		NOTE: See the Supplementary Information for more details.</li>
	</ol>
	</li>
	<li>Simultaneous electropolishing on multi-surfaces of the Al substrate
	<ol>
		<li>Pour approximate 350 mL of perchloric acid ethanol solution into double jacket beaker with maximum capability of 600 mL. Then, immerse four-fifths of the Al substrate into perchloric acid solution.</li>
		<li>Set the temperature of perchloric acid solution at 7 &#177;0.1 &#176;C using a bath circulator connected to a double jacket beaker.</li>
		<li>Clean the Al substrate through ultrasonication in acetone for 30-40 min, and rinse using acetone and D.I water a few times to remove organic residues on the surfaces of the Al substrate.</li>
		<li>Dry the Al substrate using air-gun or nitrogen (N2) gas blow for eliminating residual solvents. 
		NOTE: Natural drying under atmospheric environments is not be recommended because the solvent traces affect the electropolishing effects adversely.</li>
		<li>Connect the Al substrate working electrode (W.E.) to the positive (+) port and the platinum (Pt) wire counter electrode (C.E.) to the negative (-) port of programmable DC power supply, using alligator clips. The Al substrate and Pt wire should be parallel to each other (See the Figure S2).</li>
		<li>Apply forward bias of +20.0 V to the Al W.E. with respect to Pt C.E. for 2 - 4 min on average. Depending on the surface condition, such as contamination or roughness, the applying time could be maintained up to 5 min. Inspect all the surfaces immersed into the solution to check whether residues on the surface peel off and slide down into the solution. During this step, magnetic stirring is not recommended because inspection is difficult under stirring, and the solution flow might influence the electropolishing effect. 
		NOTE: Do not electropolish for more than 5 min, which might deteriorate the surfaces. 
		Option: Recording current-time (I-t) characteristics behavior via PC interface is helpful for monitoring the electropolishing procedure including abnormal points if they exist.</li>
		<li>Stop applying bias and disconnect the alligator clips. Pick up the Al substrate and Pt electrode carefully from the electropolishing solution. Then, remove residual solution on the surface of the Al substrate using ethanol (95%) and D.I water a few times. If the electropolishing is performed properly, mirror-like finished surfaces of the Al substrate can be identified (See the Figure S1 and Figure S3).</li>
		<li>Store the electropolished Al substrate in ethanol (95%) until the next procedure to minimize surface oxidation.</li>
	</ol>
	</li>
</ol>
3. Massive Fabrication of AAOs under Oxalic Acid Electrolyte
Note: For AAOs with a long-range arrangement of nanopores&#39; periodicity, two-step SMSAs procedure were used, in which periodically textured Al multi-surfaces were obtaining through pre-SMSA, and then, main-SMSA was conducted for fabricating the highly qualified AAOs. Repetitive application of a unit sequence can keep producing plural and almost identical AAOs until the Al substrate remains. &#34;n&#34; denotes number of the applied sequence.
<ol>
	<li>n th Pre-SMSA
	<ol>
		<li>Pour approximate 650 mL of oxalic acid aqueous solution with molar concentration of 0.3 M into a double jacket beaker with maximum capability of 1.0 L. Then, immerse about three quarters of the Al substrate into the oxalic acid solution.</li>
		<li>Set the temperature of oxalic acid electrolyte at 15 &#177;0.1 &#176;C using a bath circulator connected to a double jacket beaker.</li>
		<li>Pick up the electropolished Al substrate from ethanol, and remove the residual solvent using an air-gun or N2 gas blow.</li>
		<li>Connect the electropolished Al substrate to W.E. (+) and Pt wire to C.E. (-) of programmable DC power supply using an alligator clip. The Al substrate and Pt wire should be parallel to each other. Then, immerse the electropolished portion of the Al substrate into oxalic acid electrolyte. 
		NOTE: Make sure that enough space (e.g., approximate 1 cm) exists between the top of the acidic electrolyte and the bottom of the alligator clip connected to the Al substrate, otherwise severe corrosion occurs at the alligator clip connected position.</li>
		<li>Apply anodic bias of +40.0 V to W.E. with respect to C.E. for more than 1-2 h under moderate magnetic stirring of 100-150 rpm for maintaining electrolyte temperature. 
		NOTE: If the Pre-SMSA time is too short, multi-surfaces of the Al substrate will not be textured properly. 
		Option: Recording I-t characteristics behavior via PC interface is helpful for understanding typical behaviors in SMSA.</li>
		<li>Stop applying anodic bias after finishing pre-SMSA, and disconnect the alligator clips. Pick up the sample carefully from acidic electrolyte, and rinse the pre-SMSAed Al substrate using acetone and D.I water a few times.</li>
	</ol>
	</li>
	<li>n th Pre-AAOs etching
	<ol>
		<li>Set the temperature of chromic acid aqueous solution at 60 -&#160;65 &#176;C.</li>
		<li>Immerse the pre-SMSAed Al substrate into chromic acid solution for 1 -&#160;2 h to remove pre-AAOs on the Al substrate.</li>
		<li>Rinse the pre-AAOs removed Al substrate with acetone and D.I. water a few times. Measure the resistance of the Al substrate to confirm whether the pre-AAOs have been completely removed on the surface. If not, repeat the etching procedure again (step 3.2.2).</li>
	</ol>
	</li>
	<li>n th Main-SMSA
	<ol>
		<li>Re-set up all the experimental conditions and connections as those used in step 3.1. 
		NOTE: It should be noted that oxalic acid electrolyte can be used in a couple of sequences, and this does not influence the qualities of the main-AAOs. However, for quantitative comparisons, it is recommended that the electrolyte is used in one whole sequence, and then exchanged with a fresh one.</li>
		<li>Apply anodic bias of +40.0 V to W.E. with respect to C.E.; applying time can be varied depending on a desirable thickness of AAO. AAO growth rate was estimated to be about 8.0 and 7.5 &#956;m/h on the front and back surface of the Al substrate at the electrolyte temperature of 15 &#176;C, respectively (Refer to reference57 for more details).</li>
	</ol>
	</li>
	<li>n th SRBs-detachment
	<ol>
		<li>Stop applying the anodic bias and stirring after finishing the main-SMSA, and connect the main-SMSAed Al substrate to C.E. (-) and Pt wire to W.E. (+) of the programmable DC power supply by switching each alligator clip.</li>
		<li>Apply the SRBs, and inspect typical bubbling effects along to the multi-edges of the Al substrate covered with main-AAOs. Details of SRBs condition, such as intensity of the beginning RB, number of stairs, and duration in each stair, are closely correlated with the thickness of main-AAOs. For main-AAOs thicker than 60 &#956;m, stair in SRBs was controlled from -21 V to -24 V with the increment of -1 V and without time interval between adjacent stairs. The duration for -21 V, -22 V, and -23 V was fixed at 10 min, and the final stair of -24 V was maintained until the detaching procedure completed (See reference57 for more details including the case of thinner AAOs). 
		NOTE: It is highly recommended for a beginner to utilize PC interfaced control of SRBs and record the I-t characteristic curves during this procedure.</li>
		<li>Quit applying SRBs after finishing the detachment, and disconnect the alligator clips. Pick up the sample carefully from acidic electrolyte, and rinse them carefully with acetone and D.I water a sufficient number of times.</li>
		<li>Separate each AAO from corresponding Al surface completely. Right after step 3.4.3, upper parts of the detached AAOs are still connected to the Al substrate, which should be manually broken.</li>
	</ol>
	</li>
	<li>n th Residual alumina etching
	<ol>
		<li>Set the temperature of chromic acid solution at 60-65 &#176;C, and immerse the AAOs-detached Al substrate for about 30 min to eliminate residual alumina.</li>
		<li>Pick up the etched Al substrate, and rinse with acetone and D.I. water a few times. Measure the resistance to confirm complete removal of residual alumina. If not, repeat step 3.5.2.</li>
	</ol>
	</li>
	<li>n+1th sequence
	<ol>
		<li>Go to step 3.1, and repeat the whole sequence using the residual alumina-etched Al substrate.</li>
	</ol>
	</li>
</ol>
4. Massive Fabrication of AAOs under Sulfuric Acid Electrolyte
NOTE: In this section, clearly different conditions from those in step 3 are pointed out.
<ol>
	<li>nth Pre-SMSA
	<ol>
		<li>Pour approximate 650 mL of sulfuric acid aqueous solution (0.3 M) into a double jacket beaker with maximum capability of 1.0 L. Then, about three quarters of the Al substrate is immersed into the sulfuric acid solution.</li>
		<li>Set the temperature of the electrolyte at 0 &#177;0.1 &#176;C.</li>
		<li>Remove the residual solvent on the electropolished Al substrate using an air-gun or N2 gas blow, and connect the Al substrate to a programmable DC power supply using alligator clips (Refer to step 3.1.4)</li>
		<li>Apply anodic bias of +25.0 V to W.E. with respect to C.E. for more than 1 - 2 h under moderate magnetic stirring (100 - 150 rpm).</li>
		<li>Quit applying the anodic bias after finishing pre-SMSA, and disconnect the alligator clips. Pick up and rinse the pre-SMSAed Al substrate using acetone and D.I water a few times. 
		NOTE: For nth Pre-AAOs etching, refer to step 3.2.</li>
	</ol>
	</li>
	<li>nth Main-SMSA
	<ol>
		<li>Re-set up all the experimental conditions and connections as those used in step 4.1.</li>
		<li>Apply the same anodic bias. Applying time can be varied depending on a desirable AAO thickness. AAO growth rate was estimated to be about 5.3 &#956;m/h (Refer to reference56 for more details).</li>
	</ol>
	</li>
	<li>nth SRBs-detachment
	<ol>
		<li>Quit applying the anodic bias and stirring after finishing the main-SMSA, and connect the main-SMSAed Al substrate to C.E. (-) and Pt wire to W.E. (+) of programmable DC power supply by switching each alligator clip.</li>
		<li>Apply SRBs, and inspect typical bubbling effects along to the multi-edges of the sample. Stair in SRBs was controlled from -15 V to -17 V with the increment of -1 V and without time interval between adjacent stairs. The duration for -15 V and -16 V was fixed at 10 min, and final stair of -17 V was maintained until the detaching procedure completed. 
		NOTE: Based on the more fragile nature of AAOs fabricated under sulfuric acid electrolyte, the current level was abruptly increased at the detaching moments accompanied with noticeable clicking sounds.</li>
		<li>Quit applying the SRBs after finishing the detachment, and disconnect the alligator clips. Pick up the sample carefully from the acidic electrolyte, and rinse carefully with acetone and D.I water a sufficient number of times.</li>
		<li>Separate each AAO from the corresponding Al surface mechanically by breaking the upper parts of as-detached AAOs. 
		NOTE: For nth Residual alumina etching refer to step 3.5.</li>
	</ol>
	</li>
	<li>n+1 th sequence
	<ol>
		<li>Go to step 4.1, and repeat the whole sequence using the residual alumina-etched Al substrate.</li>
	</ol>
	</li>
</ol>

This research was supported in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2016R1C1B1016344 and 2016R1E1A2915664).

In this paper, we successfully demonstrated a facile, high yield, and environmentally clean method to fabricate nanoporous AAOs through SMSA and SRBs-detachment, which could be repeated to the same Al substrate for significantly enhancing mass-producibility as well as usability of limited natural resource. As shown in the flow chart of Figure 1a, our AAO fabricating strategy is based on the conventional two-step anodization, which was modified on multi-surfaces situation. Individual procedures functioned well independent of the other surfaces, because electric fields in the electropolishing and two-step SMSAs procedures were formed in the normal directions on the multi-surfaces, where the electrochemical reaction occurs simultaneously. In this point of view, position of each surface and corresponding AAO will be defined with respect to the counter electrode, as shown in Figure 1b; e.g., &#34;Front&#34; designate a surface confronting the Pt counter electrode, and so on.
Pristine Al substrate showed rougher surfaces due to mechanical polishing, which became much smoother after electropolishing procedure. Each surface of the electropolished Al substrate looked like a mirror in macroscale, however, it was covered with irregularly distributed nanoscale concaves as shown in the inset (ii) of Figure 1a. Therefore, not only every cleaning but also drying treatment were also very important, owing to the fact that solvent traces could significantly affect surface morphologies in procedures after electropolishing. Once deteriorated, surfaces never recovered, and kept the poor morphologies. In this regard, excessive electropolishing treatment would not be good either. If electropolishing time is too long, periodically arranged wavy valleys were formed on the entire Al surfaces, which could increase an adhesive strength between AAOs and Al. A unit sequence depicted by a blue-dashed box shown in Figure 1a consists of nth pre-SMSA, nth pre-AAOs etching, nth main-SMSA, nth SRBs-detachment, and nth residual alumina etching, where n is number of the applied sequence (n = 1, 2, 3, &#8230;).
Figure 2 compares the I-t characteristic curves of pre-/main-SMSA and SRBs-detachment from 1st to 5th sequences. In both SMSAs, the current level gradually decreased with increasing applying time. These typical features were only observed in a multi-surfaces situation attributing to the gradual reduction of total anodizing area as well as accumulation of mechanical stresses due to the viscous flow23,58 and volume expansion23,59,60,61,62 during simultaneous formations of plural AAOs56,57. Previous reports on these SMSA and SRBs-detachment proposed the stress-released direct detaching mechanism, which could be further optimized through appropriate SRBs conditions for relatively thinner thickness of AAO (Refer to reference57 for more details).
An intuitive schematic illustration implying massive producibility is successfully realized in Figure 3 and Figure 4 exhibiting results of about total 5th times iterations of the unit sequence under oxalic and sulfuric acid electrolyte, respectively. Each photograph clearly shows that all the AAOs having the exact equal dimensions to those of corresponding front and back surfaces (See the Supplementary Information for the AAOs detached from sides and bottom surfaces). Barrier side SEM images of all sequences indicate that cleavage planes are beneath the barrier oxides in both acidic electrolytes, which are similar results about cathodic delamination of a relatively thicker AAO on mono-surface50,51. As an alternative approach for obtaining AAO with through-hole structures (i.e., without barrier oxide), anodic voltage pulse detachment using another detaching electrolyte7,41,42,52,53,54,55 or two-layer anodization incorporating normal AAO into a sacrificial one fabricated from acidic electrolyte of extremely high concentration (12.0 M)63 might be taken into consideration.
The SMSA and SRBs-based strategy appears to possess an acid-type independent nature, therefore, its various advantages and strengths are worth expanding into phosphoric acid electrolyte and/or HA condition, which will enrich the potentials of nanoporous AAOs toward more versatile applications.

AAOs which were formed by anodizing Al substrate in an acidic electrolyte, have attracted great interest in diverse fundamental science and industry, for example, hard templates for nanotubes/nanowires1,2,3,4,5, energy storage devices6,7,8,9, bio-sensing10,11, filtering applications12,13,14, masks for evaporating and/or etching15,16,17, and capacitive humidity sensors18,19,20,21,22, owing to their self-ordered honeycomb structure, high aspect ratio of nanopores, and superior mechanical properties23. For applying the nanoporous AAOs to these various applications, they should be freestanding forms with a highly and long-range ordered array of nanopores. In this regard, strategies for obtaining AAOs must consider both formation (anodizing) and separation (detaching) procedures.
In the viewpoint of the AAO formation, mild anodization (hereafter referred as MA) was well established under sulfuric, oxalic, and phosphoric acidic electrolytes23,24,25,26,27. However, MA processes exhibited low-yields of AAO fabrication due to their slow growth rate depending on relatively low intensities of anodic voltages, which would further deteriorate through a two-step MA process for improving nanopores&#39; periodicity28,29. Thus, hard anodization (HA) techniques were proposed as alternatives of MA by applying higher anodic voltages (oxalic/sulfuric acid electrolyte) or using more concentrated electrolyte (phosphoric acid)30,31,32,33,34,35,36,37,38,39,40. HA processes show distinct enhancements of growth rates as well as periodic arrangements, whereas resulting AAOs became more fragile, and the density of nanopores were reduced30. In addition, an expensive cooling system is required for dissipating Joule&#39;s heating caused by high current density31. These results restrict the potential applicability of the AAOs via HA processes.
For separating an AAO from the corresponding surface of Al plate, selective chemical etching of the remaining Al substrate was most widely utilized in both the MA and HA processes using toxic chemicals, such as copper chloride35,39,41,42 or mercury chloride16,17,43,44,45,46,47,48,49. However, this method induces disadvantageous side effects, e.g., a longer reaction time proportional to the remaining thickness of the Al, contamination of AAO by heavy metal ions, harmful residues to human body/natural environments, and inefficient usage of valuable resources. Therefore, many attempts have been made for realizing direct detachment of an AAO. Although both cathodic voltage delamination50,51 and anodic voltage pulse detachment7,41,42,52,53,54,55 present a merit that the remaining Al substrate can be reused, the former technique takes almost comparable time with those in chemical etching methods50. Notwithstanding clear reduction of the processing time, harmful and highly reactive chemicals, for examples butanedione and/or perchloric acid, were used as detaching electrolytes in the latter techniques55, where an additional cleaning procedure is needed because of the changing electrolyte between the anodizing and detaching procedure. Especially, the detaching behaviors and quality of the detached AAOs severely influence the thickness. In the case of the AAO with relatively thinner thickness, the detached one might contain cracks and/or apertures.
All the experimental approaches listed above have been applied to a &#34;single-surface&#34; of the Al specimen, excluding surface protecting/engineering purposes, and this feature of the conventional technologies exhibits critical limitations of the AAO fabrication in terms of yield as well as processibility, which also influences the potential applicability of the AAOs56,57.
To satisfy the increasing demands in the AAO-related fields in terms of facile, high yield, and green technological approaches, we previously reported on SMSA and direct detachment through SRBs under sulfuric56 and oxalic57 acid electrolyte, respectively. It is a well-known fact that plural AAOs can be formed on the multiple surfaces of the Al substrate immersed into acidic electrolytes. However, SRBs, a key distinction of our methods, enable the detachment of those AAOs from the corresponding multi-surfaces of the Al substrate in the same acidic electrolyte used for the SMSAs indicating mass-production, simplicity, and green technological characteristics. We would like to point out that SRBs-based detachment is an optimal strategy for plural AAOs fabricated by SMSAs56,57 and even valid for relatively thinner thicknesses of AAOs57 when compared with cathodic delamination (i.e., constant reverse bias) on single-surface51. Finally, a unit sequence consisting of the SMSAs sequentially combined with SRBs-based detachment can be applied repeatedly to the same Al substrate, avoiding complicated procedures and toxic/reactive chemicals, which reinforces the advantages of our strategies and also guarantees the efficient usage of natural resources.

<table><tbody><tr><td>Sulfuric Acid &gt;98%</td><td>DUKSAN reagent</td><td>5950</td><td></td></tr><tr><td>Oxalic Acid Anhydrous, 99.5-100.2%</td><td>KANTO chemical</td><td>31045-73</td><td></td></tr><tr><td>Phosphoric Acid, 85%</td><td>SAMCHUN chemical</td><td>P0463</td><td></td></tr><tr><td>Perchloric Acid, 60%</td><td>SAMCHUN chemical</td><td>P0181</td><td>Highly Reactive</td></tr><tr><td>Chromium(VI) Oxide</td><td>Sigma Aldrich</td><td>232653</td><td>Strong Oxidizer</td></tr><tr><td>Ethanol, 95%</td><td>SAMCHUN chemical</td><td>E0219</td><td></td></tr><tr><td>Absolute Ethanol, 99.9%</td><td>SAMCHUN chemical</td><td>E1320</td><td></td></tr><tr><td>Double Jacket Beaker</td><td>iNexus</td><td>27-00292-05</td><td></td></tr><tr><td>Low Temperature Bath Circulator</td><td>JEIO TECH</td><td>AAH57052K</td><td></td></tr><tr><td>Programmable DC Power Supply</td><td>PNCYS</td><td>EDP-3001&amp;nbsp;</td><td></td></tr><tr><td>Aluminum Plate, &gt;99.99%</td><td>Goodfellow</td><td></td><td></td></tr><tr><td>Platinum Cylinder</td><td>Whatman</td><td>444685</td><td></td></tr><tr><td>Pure &amp;amp; Ultra Pure Water System (Deionized Water)</td><td>Human Science</td><td>Pwer II &amp;amp; HIQ II</td><td></td></tr></tbody></table>

simultaneous multi-surface anodizations and stair-like reverse biases detachment of anodic aluminum oxides in sulfuric and oxalic acid electrolyte

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental sciences and industrial applications owing to their periodic arrangement of nanopores with relatively high aspect ratio. However, the techniques reported so far, which could be only valid for mono-surface anodization, show critical disadvantages, i.e., time-consuming as well as complicated procedures, requiring toxic chemicals, and wasting valuable natural resources. In this paper, we demonstrate a facile, efficient, and environmentally clean method to fabricate nanoporous AAOs in sulfuric and oxalic acid electrolytes, which can overcome the limitations that result from conventional AAO fabricating methods. First, plural AAOs are produced at one time through simultaneous multi-surfaces anodization (SMSA), indicating mass-producibility of the AAOs with comparable qualities. Second, those AAOs can be separated from the aluminum (Al) substrate by applying stair-like reverse biases (SRBs) in the same electrolyte used for the SMSAs, implying simplicity and green technological characteristics. Finally, a unit sequence consisting of the SMSAs sequentially combined with SRBs-based detachment can be applied repeatedly to the same Al substrate, which reinforces the advantages of this strategy and also guarantees the efficient usage of natural resources.

Flow chart of nth AAO fabricating sequence mainly consisting of two-step SMSAs, SRBs-detachment, and related chemical etching was presented schematically in Figure 1a. Each inset show a scanning electron microscope (SEM) image of the corresponding surface morphology at each individual procedure and a photograph taken immediately after SRBs-detachment. A schematic illustration after the total 5th repetition of the unit sequence exhibited advantages of SMSA and SRBs-based strategies (Figure 1b). I-t characteristic curves of the pre- and main-SMSAs up to the 5th sequences were compared in Figure 2a and Figure 2b, respectively. A comparison of I-t characteristic curves from each SRBs-detaching procedure are shown in Figure 2c. Photograph and corresponding SEM images of the main-AAOs obtained from front and back surfaces under oxalic and sulfuric acid electrolytes are presented in Figure 3 and Figure 4, respectively.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/56432/56432fig1.jpg" /> 
Figure 1:&#160;nth AAOs fabrication procedures (n = 1, 2, 3 &#8230;). (a) Schematic flow chart including corresponding SEM images in nth AAOs fabricating sequence: (i) Pristine Al substrate, (ii) Electro polishing, (iii) nth pre-SMSA, (iv) nth pre-AAOs etching, (v) nth main-SMSA, (vi) nth SRBs-detaching, (vii) nth residual alumina etching. A unit sequence was depicted using blue-dashed box. (b) Schematic illustration showing that plural AAOs with equal dimensions of corresponding surfaces were successfully obtained from multi-surfaces of a single Al plate through 5th repetitive applications of the unit sequence. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56432/56432fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/56432/56432fig2.jpg" /> 
Figure 2: Peculiar behaviors during two-step SMSAs and SRBs-detachments of AAOs under oxalic acid electrolyte at 15 &#176;C. I-t characteristic curves of (a) pre- and (b) main-SMSAs from 1st to 5th sequences, respectively. (c) I-t characteristic curves of SRBs-detaching procedures from 1st to 5th sequences. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56432/56432fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/56432/56432fig3.jpg" /> 
Figure 3: Photograph of the remaining Al substrate and main-AAOs after 5th repetitive applications of the unit sequence under oxalic acid electrolyte. AAOs obtained from front and back surfaces were distinguished by red- and blue-dashed boxes, respectively. Insets: Open-pore and barrier side SEM images of the corresponding 1st to 5th main-AAOs. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56432/56432fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/56432/56432fig4.jpg" /> 
Figure 4:Photograph of the remaining Al substrate and main-AAOs after 5th repetitive applications of the unit sequence under sulfuric acid electrolyte. AAOs obtained from front and back surfaces were distinguished by red- and blue-dashed boxes, respectively. Insets: Open-pore and barrier side SEM images of the corresponding 1st to 5th main-AAOs. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56432/56432fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplementary Information: <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/56432/Supplementary_Information.docx" target="_blank">Please click here to download this file.</a>

Watch this Scientific Journal Video about Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte at JoVE.com

Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

A protocol for fabricating nanoporous anodic aluminum oxides via simultaneous multi-surfaces anodization followed by stair-like reverse biases detachments is presented. It can be applied repeatedly to the same aluminum substrate, exhibiting a facile, high-yield, and environmentally clean strategy.

After reporting on the two-step anodization, nanoporous anodic aluminum oxides (AAOs) have been widely utilized in the versatile fields of fundamental ...

simultaneous-multi-surface-anodizations-stair-like-reverse-biases

Nanyang Technological University

Research

JoVE Journal

Engineering

7.2K Views.  Sungkyunkwan University. Een protocol voor het fabriceren van nanoporeuze anodic aluminium stikstofoxiden via gelijktijdig meerdere oppervlakken anodisatie gevolgd door trap-achtige omgekeerde vertekeningen detachementen wordt gepresenteerd. Het kan herhaaldelijk worden toegepast op de dezelfde aluminium substraat, een facile tentoonstellen, hoge opbrengst en ecologisch schoon strategie.

Video: Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

Ultrahigh Density Array of Vertically Aligned Small-molecular Organic Nanowires on Arbitrary Substrates

In recent years &pi;-conjugated organic semiconductors have emerged as the active material in a number of diverse applications including large-area, low-cost displays, photovoltaics, printable and flexible electronics and organic spin valves. Organics allow (a) low-cost, low-temperature processing and (b) molecular-level design of electronic, optical and spin transport characteristics. Such features are not readily available for mainstream inorganic semiconductors, which have enabled organics to carve a niche in the silicon-dominated electronics market. The first generation of organic-based devices has focused on thin film geometries, grown by physical vapor deposition or solution processing. However, it has been realized that organic nanostructures can be used to enhance performance of above-mentioned applications and significant effort has been invested in exploring methods for organic nanostructure fabrication.	
A particularly interesting class of organic nanostructures is the one in which vertically oriented organic nanowires, nanorods or nanotubes are organized in a well-regimented, high-density array. Such structures are highly versatile and are ideal morphological architectures for various applications such as chemical sensors, split-dipole nanoantennas, photovoltaic devices with radially heterostructured "core-shell" nanowires, and memory devices with a cross-point geometry. Such architecture is generally realized by a template-directed approach. In the past this method has been used to grow metal and inorganic semiconductor nanowire arrays. More recently &pi;-conjugated polymer nanowires have been grown within nanoporous templates. However, these approaches have had limited success in growing nanowires of technologically important &pi;-conjugated small molecular weight organics, such as tris-8-hydroxyquinoline aluminum (Alq3), rubrene and methanofullerenes, which are commonly used in diverse areas including organic displays, photovoltaics, thin film transistors and spintronics.	
Recently we have been able to address the above-mentioned issue by employing a novel "centrifugation-assisted" approach. This method therefore broadens the spectrum of organic materials that can be patterned in a vertically ordered nanowire array. Due to the technological importance of Alq3, rubrene and methanofullerenes, our method can be used to explore how the nanostructuring of these materials affects the performance of aforementioned organic devices. The purpose of this article is to describe the technical details of the above-mentioned protocol, demonstrate how this process can be extended to grow small-molecular organic nanowires on arbitrary substrates and finally, to discuss the critical steps, limitations, possible modifications, trouble-shooting and future applications.

We report a simple method for fabricating an ultrahigh density array of vertically ordered small-molecular organic nanowires. This method allows for synthesis of complex heterostructured hybrid nanowire geometries, which can be inexpensively grown on arbitrary substrates. These structures have potential applications in organic electronics, optoelectronics, chemical sensing, photovoltaics and spintronics.

In  recent years &pi;-conjugated  organic semiconductors have emerged as the active material in a number of  diverse applications including large-area, ...

Real-Time DC-dynamic Biasing Method for Switching Time Improvement in Severely Underdamped Fringing-field Electrostatic MEMS Actuators

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input bias voltage. However, the tradeoff for the improved switching performance is a relatively long settling time to reach each gap height in response to various applied voltages. Transient applied bias waveforms are employed to facilitate reduced switching times for electrostatic fringing-field MEMS actuators with high mechanical quality factors. Removing the underlying substrate of the fringing-field actuator creates the low mechanical damping environment necessary to effectively test the concept. The removal of the underlying substrate also a has substantial improvement on the reliability performance of the device in regards to failure due to stiction. Although DC-dynamic biasing is useful in improving settling time, the required slew rates for typical MEMS devices may place aggressive requirements on the charge pumps for fully-integrated on-chip designs. Additionally, there may be challenges integrating the substrate removal step into the back-end-of-line commercial CMOS processing steps. Experimental validation of fabricated actuators demonstrates an improvement of 50x in switching time when compared to conventional step biasing results. Compared to theoretical calculations, the experimental results are in good agreement.

The robust device design of fringing-field electrostatic MEMS actuators results in inherently low squeeze-film damping conditions and long settling times when performing switching operations using conventional step biasing. Real-time switching time improvement with DC-dynamic waveforms reduces the settling time of fringing-field MEMS actuators when transitioning between up-to-down and down-to-up states.

Mechanically underdamped electrostatic fringing-field MEMS actuators are well known for their fast switching operation in response to a unit step input ...

A Method to Manipulate Surface Tension of a Liquid Metal via Surface Oxidation and Reduction

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where interfacial tension is a dominant force. A variety of methods exist for controlling the interfacial tension of aqueous and organic liquids on this scale; however, these techniques have limited utility for liquid metals due to their large interfacial tension.
Liquid metals can form soft, stretchable, and shape-reconfigurable components in electronic and electromagnetic devices. Although it is possible to manipulate these fluids via mechanical methods (e.g., pumping), electrical methods are easier to miniaturize, control, and implement. However, most electrical techniques have their own constraints: electrowetting-on-dielectric requires large (kV) potentials for modest actuation, electrocapillarity can affect relatively small changes in the interfacial tension, and continuous electrowetting is limited to plugs of the liquid metal in capillaries.
Here, we present a method for actuating gallium and gallium-based liquid metal alloys via an electrochemical surface reaction. Controlling the electrochemical potential on the surface of the liquid metal in electrolyte rapidly and reversibly changes the interfacial tension by over two orders of magnitude ( &#820;500 mN/m to near zero). Furthermore, this method requires only a very modest potential (&#60; 1 V) applied relative to a counter electrode. The resulting change in tension is due primarily to the electrochemical deposition of a surface oxide layer, which acts as a surfactant; removal of the oxide increases the interfacial tension, and vice versa. This technique can be applied in a wide variety of electrolytes and is independent of the substrate on which it rests.

We present a method to control the interfacial energy of a liquid metal in an electrolyte via electrochemical deposition (or removal) of a surface oxide layer. This simple method can control the capillary behavior of gallium-based liquid metals by tuning the interfacial energy rapidly, significantly, and reversibly using modest voltages.

Controlling interfacial tension is an effective method for manipulating the shape, position, and flow of fluids at sub-millimeter length scales, where ...

Chemistry

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures

We fabricate metal/LaAlO3/SrTiO3 heterostructures using a combination of pulsed laser deposition and in situ magnetron sputtering. Through magnetotransport and in situ X-ray photoelectron spectroscopy experiments, we investigate the interplay between electrostatic and chemical phenomena of the quasi two-dimensional electron gas formed in this system.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide ...

Multiscale Structures Aggregated by Imprinted Nanofibers for Functional Surfaces

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing challenge in the field is the fabrication of hybrid micro-nano structures using a facile, cost-effective, and high-throughput method. To overcome these challenges, this paper proposes a protocol to fabricate multiscale structures using only an imprint process with an anodic aluminum oxide (AAO) filter and an evaporative self-aggregation process of nanofibers. Unlike previous attempts that have aimed to straighten nanofibers, we demonstrate a unique fabrication method for multiscale aggregated nanofibers with high aspect ratios. Furthermore, the surface morphology and wettability of these structures on various liquids were investigated to facilitate their use in multifunctional surfaces.

Presented is an easy method to fabricate nano-micro multiscale structures, for functional surfaces, by aggregating nanofibers fabricated using an anodic aluminum oxide filter.

Multiscale surface structures have attracted increasing interest owing to several potential applications in surface devices. However, an existing ...

Scalable Stamp Printing and Fabrication of Hemiwicking Surfaces

Hemiwicking is a process where a fluid wets a patterned surface beyond its normal wetting length due to a combination of capillary action and imbibition. This wetting phenomenon is important in many technical fields ranging from physiology to aerospace engineering. Currently, several different techniques exist for fabricating hemiwicking structures. These conventional methods, however, are often time consuming and are difficult to scale-up for large areas or are difficult to customize for specific, nonhomogeneous patterning geometries. The presented protocol provides researchers with a simple, scalable, and cost-effective method for fabricating micro-patterned hemiwicking surfaces. The method fabricates wicking structures through the use of stamp printing, polydimethylsiloxane (PDMS) molding, and thin-film surface coatings. The protocol is demonstrated for hemiwicking with ethanol on PDMS micropillar arrays coated with a 70 nm thick aluminum thin-film.

A simple protocol is provided for the fabrication of hemiwicking structures of varying sizes, shapes, and materials. The protocol uses a combination of physical stamping, PDMS molding, and thin-film surface modifications via common materials deposition techniques.

Hemiwicking is a process where a fluid wets a patterned surface beyond its normal wetting length due to a combination of capillary action and imbibition. ...

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping microtextures (GEMs) on them comprising cavities and pillars with reentrant and doubly reentrant features. Specifically, we use SiO2/Si as the model system and share protocols for two-dimensional (2D) designing, photolithography, isotropic/anisotropic etching techniques, thermal oxide growth, piranha cleaning, and storage towards achieving those microtextures. Even though the conventional wisdom indicates that roughening intrinsically wetting surfaces (&#952;o &#60; 90&#176;) renders them even more wetting (&#952;r &#60; &#952;o &#60; 90&#176;), GEMs demonstrate liquid repellence despite the intrinsic wettability of the substrate. For instance, despite the intrinsic wettability of silica &#952;o &#8776; 40&#176; for the water/air system, and &#952;o &#8776; 20&#176; for the hexadecane/air system, GEMs comprising cavities entrap air robustly on immersion in those liquids, and the apparent contact angles for the droplets are &#952;r &#62; 90&#176;. The reentrant and doubly reentrant features in the GEMs stabilize the intruding liquid meniscus thereby trapping the liquid-solid-vapor system in metastable air-filled states (Cassie states) and delaying wetting transitions to the thermodynamically-stable fully-filled state (Wenzel state) by, for instance, hours to months. Similarly, SiO2/Si surfaces with arrays of reentrant and doubly reentrant micropillars demonstrate extremely high contact angles (&#952;r &#8776; 150&#176;&#8211;160&#176;) and low contact angle hysteresis for the probe liquids, thus characterized as superomniphobic. However, on immersion in the same liquids, those surfaces dramatically lose their superomniphobicity and get fully-filled within &#60;1 s. To address this challenge, we present protocols for hybrid designs that comprise arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles. Indeed, hybrid microtextures entrap air on immersion in the probe liquids. To summarize, the protocols described here should enable the investigation of GEMs in the context of achieving omniphobicity without chemical coatings, such as perfluorocarbons, which might unlock the scope of inexpensive common materials for applications as omniphobic materials. Silica microtextures could also serve as templates for soft materials.

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

This work presents microfabrication protocols for achieving cavities and pillars with reentrant and doubly reentrant profiles on SiO2/Si wafers using photolithography and dry etching. Resulting microtextured surfaces demonstrate remarkable liquid repellence, characterized by robust long-term entrapment of air under wetting liquids, despite the intrinsic wettability of silica.

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping ...

The Effect of Anodization Parameters on the Aluminum Oxide Dielectric Layer of Thin-Film Transistors

Aluminum-oxide (Al2O3) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the dielectric layer of thin-film transistors (TFTs). Growth of aluminum-oxide layers from anodization of metallic aluminum films is greatly advantageous when compared to sophisticated processes such as atomic layer deposition (ALD) or deposition methods that demand relatively high temperatures (above 300 &#176;C) such as aqueous combustion or spray-pyrolysis. However, the electrical properties of the transistors are highly dependent on the presence of defects and localized states at the semiconductor/dielectric interface, which are strongly affected by the manufacturing parameters of the anodized dielectric layer. To determine how several fabrication parameters influence the device performance without performing all possible combination of factors, we used a reduced factorial analysis based on a Plackett-Burman design of experiments (DOE). The choice of this DOE permits the use of only 12 experimental runs of combinations of factors (instead of all 256 possibilities) to obtain the optimized device performance. The ranking of the factors by the effect on device responses such as the TFT mobility is possible by applying analysis of variance (ANOVA) to the obtained results.

Anodization parameters for growth of the aluminum-oxide dielectric layer of zinc-oxide thin-film transistors (TFTs) are varied to determine the effects on the electrical parameter responses. Analysis of variance (ANOVA) is applied to a Plackett-Burman design of experiments (DOE) to determine the manufacturing conditions that result in optimized device performance.

Aluminum-oxide (Al2O3) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the ...

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different materials show variability in resistive switching properties due to the intrinsic nature of the material used, leading to discrepancies in the field because of underlying operation mechanisms. This highlights a need for a reliable technique to understand mechanisms using nanostructural observations. This protocol explains a detailed process and methodology of in&#160;situ nanostructural analysis as a result of electrical biasing using transmission electron microscopy (TEM). It provides visual and reliable evidence of underlying nanostructural changes in real time memory operations. Also included is the methodology of fabrication and electrical characterizations for asymmetric crossbar structures incorporating amorphous vanadium oxide. The protocol explained here for vanadium oxide films can be easily extended to any other materials in a metal-dielectric-metal sandwiched structure. Resistive switching crossbars are predicted to serve the programmable logic and neuromorphic circuits for next-generation memory devices, given the understanding of the operation mechanisms. This protocol reveals the switching mechanism in a reliable, timely, and cost-effective way in any type of resistive switching materials, and thereby predicts the device&#39;s applicability.

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VOx

Presented here is a protocol for analyzing nanostructural changes during in&#160;situ biasing with transmission electron microscopy (TEM) for a stacked metal-insulator-metal structure. It has significant applications in resistive switching crossbars for the next generation of programmable logic circuits and neuromimicking hardware, to reveal their underlying operation mechanisms and practical applicability.

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different ...

Versatile Technique to Produce a Hierarchical Design in Nanoporous Gold

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, drug loading and release, and the development of catalysts have unquestionably accelerated the usage of nanoporous gold (NPG)-based nanomaterials in research and development. This article describes the process of the generation of hierarchical bimodal nanoporous gold (hb-NPG) by employing a step-wise procedure involving electrochemical alloying, chemical dealloying techniques, and annealing to create both macro- and mesopores. This is done to improve the utility of NPG by creating a bicontinuous solid/void morphology. The area available for surface modification is enhanced by smaller pores, while molecular transport benefits from the network of larger pores. The bimodal architecture, which is the result of a series of fabrication steps, is visualized using scanning electron microscopy (SEM) as a network of pores that are less than 100 nm in size and connected by ligaments to larger pores that are several hundred nanometers in size. The electrochemically active surface area of the hb-NPG is assessed using cyclic voltammetry (CV), with a focus on the critical roles that both dealloying and annealing play in creating the necessary structure. The adsorption of different proteins is measured by solution depletion technique, revealing the better performance of hb-NPG in terms of protein loading. By changing the surface area to volume ratio, the created hb-NPG electrode offers tremendous potential for biosensor development. The manuscript discusses a scalable method to create hb-NPG surface structures, as they offer a large surface area for the immobilization of small molecules and improved transport pathways for faster reactions.

Nanoporous gold with a hierarchical and bimodal pore size distribution can be produced by combining electrochemical and chemical dealloying. The composition of the alloy can be monitored via EDS-SEM examination as the dealloying process advances. The material's loading capacity can be determined by studying protein adsorption onto the material.

The potential to generate variable pore sizes, simplistic surface modification, and a breadth of commercial uses in the fields of biosensors, actuators, ...

Simultaneous Multi-surface Anodizations and Stair-like Reverse Biases Detachment of Anodic Aluminum Oxides in Sulfuric and Oxalic Acid Electrolyte

Samenvatting

Meer video's verkennen

Hoofdstukken in deze video