The influence of electrolyte composition on the growth of nanoporous anodic alumina

The influence of electrolyte composition on the growth of nanoporous anodic alumina

Electrochimica Acta 211 (2016) 453–460 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/elect...

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Electrochimica Acta 211 (2016) 453–460

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

The influence of electrolyte composition on the growth of nanoporous anodic alumina Wojciech J. Ste˛pniowskia,* , Marcin Monetaa , Małgorzata Noreka ,  skab , Alice Scarpellinic , Marco Salernod Marta Michalska-Doman a Department of Advanced Materials and Technologies, Faculty of Advanced Technologies and Chemistry, Military University of Technology, Kaliskiego 2 Str., 00-908 Warszawa, Poland b Institute of Optoelectronics, Military University of Technology, Kaliskiego 2 Str., 00-908 Warszawa, Poland c Department of Nanochemistry, Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy d Department of Nanophysics, Istituto Italiano di Tecnologia, via Morego 30, Genova 16163, Italy

A R T I C L E I N F O

Article history: Received 19 April 2016 Received in revised form 20 May 2016 Accepted 15 June 2016 Available online 16 June 2016 Keywords: Anodization Self-organization Alumina Nanopores Ionic mobility

A B S T R A C T

Aluminum was anodized in mixtures of aqueous sulfuric and chromic acid in different ratios, with overall concentration of 1.0 M. It was found that the logarithm of current density (and consequently, oxide growth rate) is a square function of the anodizing voltage. Moreover, the barrier layer thickness at the pores bottom was found to increase exponentially with the voltage, and increased as well with the fraction of chromic acid in the electrolyte. Additionally, interpore distance of anodic aluminum oxide, formed at the same voltage, was found to increase exponentially with the molar fraction of the chromic acid. Altogether, the high impact of the composition of the electrolyte on the morphological features of the nanoporous arrays is revealed. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Anodization of aluminum gives numerous opportunities to researchers in the field of nanofabrication. Recently, several advances in catalysis [1], optics [2], fuels cells assembly [3,4], energy storage [5], sensors performance [6,7], surface enhanced Raman spectroscopy (SERS) [8,9], magnetic materials engineering [10,11], biomaterials engineering [12], drug-releasing platforms [13,14], structural color generation [15–17] and fabrication of hierarchical 3D nanostructures [18], were achieved with the use of anodic aluminum oxide (AAO). To form nanoporous AAO, aluminum is oxidized in acidic electrolytes at relevant voltage range, determined by the type of the electrolyte. Geometrical features of AAO, like pore diameter, interpore distance and thickness of the formed anodic oxide are controlled by the operating conditions like type, concentration and temperature of the electrolyte [19–24], applied voltage [19–24] and duration of the second step of anodization [19–24].

* Corresponding author. Tel.: +48 261 83 94 46; fax: +48 261 83 94 45. E-mail address: [email protected] (W.J. Ste˛pniowski). http://dx.doi.org/10.1016/j.electacta.2016.06.076 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.

Recently, to widen the area of experimental conditions, new, organic and inorganic, electrolytes are being applied [15,17,25,26], as well as new approaches like pulse anodizing [16] or sinusoidal anodizing [27]. Moreover, various additives are being introduced to the electrolyte to form AAO at lower voltages [28], or with higher oxide growth rate [29]. Additionally, anodization in nonaqueous electrolytes is also performed [30,31]. It was found that the higher the viscosity of the electrolyte the larger the interpore distance and the lower the oxide growth rate [32]. It was found that ionic mobility plays important role in the AAO growth [32]. For the same reason, anodizations in mixed electrolytes are being performed, e.g. sulfuric acid with oxalic acid [33–36], oxalic acid with phosphoric acid [37], and phosphoric acid with citric acid and ethylene glycol [38]. All these anodization strategies mentioned above allowed to form AAO with nanopores significantly varied in size: from about 10 nm in diameter up to micron scale [38]. Despite the anodization in mixed electrolytes has been already reported, until now no in-depth study of the influence of the mixed electrolyte composition on the AAO growth has been presented. Moreover, it is suspected that various compositions of the electrolyte may have similar impact on the anodic oxide growth,

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in terms of current density, oxide growth rate and morphological features, as viscosity, while both viscosity and average hydrodynamic radius of the electrolyte are inversely proportional to the ionic mobility, and linked with current density [32]. In this work, the influence of electrolyte composition on the anodic oxide growth phenomena is systematically researched in detail. Sulfuric acid and chromic acid mixtures with various molar ratio were chosen to anodized aluminum. Both electrolytes are applied in industrial practice and fundamental research as well. 2. Experimental High purity (99.9995%), 0.25 mm thick Al foil (Alfa-Aesar, Puratronic) was cut into samples (25 mm  10 mm), degreased (in acetone and ethanol subsequently) and electropolished (in a mixture of perchloric acid and ethanol, HClO4:C2H5OH 1:4 vol., at 0  C and 20 V for 2 min). The so-prepared samples were protected with acid-resistant non-conductive paint at back and edges, such that the working surface area was limited to 1.0 cm2. The samples were then anodized for 12 h at 0  C in aqueous electrolytes containing sulfuric and/or chromic acid of various molar proportions, accordingly to Table 1. To prevent aluminum samples anodized in sulfuric acid from “burning”, temperature equal 0  C was applied [39,40] The samples were anodized at voltage ranging from 15 to 60 V with 5 V steps. The upper voltage according to a set limitation against too high current densities j, occurring for the most aggressive electrolyte compositions, which would otherwise cause “burning” of the anode [39,40]. After anodization, the formed oxide was chemically removed in a mixture of 6 wt.% H3PO4 and 1.8 wt.% H2CrO4 at 60  C for 90 min. Subsequently, the samples were re-anodized at the same operating conditions as during the first anodization, to obtain highly-ordered nanoporous AAO. Characterization of the AAO morphology was done with scanning electron microscope (SEM) imaging made with fieldemission (FE) SEM instrument Quanta D FEG (FEI, USA). The oxide growth rate d was estimated from the FE-SEM cross sectional images of AAO. To obtain average pore diameter, 3 FE-SEM images from each sample were taken and analyzed with NIS-Elements software. Depending on the operating conditions, approximately 2000 of pores per sample were analyzed and 3s test was applied. To estimate average interpore distance, 3 FE-SEM images from each sample were taken and analyzed with WSxM software [41,42]. Radial averages of fast Fourier transform of each image were calculated. The inverted value of the abscissa of the radial average maximum equals interpore distance. 3. Results and discussion Fig. 1 shows the first 50 min of the second step of anodization at 25.0 V. It is clearly seen that the higher the molar fraction of chromic acid xCrx the lower the current density j. This is caused by the decrease in the ionic mobility of the electrolyte with the

Fig. 1. First 3000 seconds of the second step anodization performed at 25.0 V in electrolytes with various values of the molar fraction of chromic acid.

increase of chromate content. In fact, the hydrodynamic radius a of the sulfate anion is smaller than that of the chromate anion, what has direct impact on their mobility u, according to [32,43]: u¼

ze 6pha

ð1Þ

where e is elemental charge, z is the ion charge in e units, and h is electrolyte viscosity. Additionally, there is a linear relation between the ionic mobility u and the current density j [32,43]: j ¼ aco eðzþ uþ þ z u ÞE

ð2Þ

where a is the dissociation fraction, c0 the concentration, z+,- the ion charge coefficient, u+,-the ionic mobility of cation and anion respectively, and E the electric field intensity. As can be seen in Eq. (2), not only the ionic mobility has a great impact on the current density, but also the dissociation fraction of the electrolyte has. H2SO4 is considered as strong acid, while H2CrO4 has a pKa1 of 0.74, which means a much lower dissociation fraction, and this is another reason for the decrease in current density j with the molar fraction xCr. Additionally, for mixed electrolytes, especially for those with lower pH, CrO42 anions may transform into Cr2O72 anions (or in larger ones) with bigger hydrodynamic radius and lower mobility, what additionally decreases the current density for chromate-rich electrolytes.

Table 1 Chemical composition of the electrolytes and applied voltage range. Concentration of sulfuric acid/M

Concentration of chromic acid/M

Molar fraction of chromic acid xCr

Applied voltage range/V

1.0 0.8 0.6 0.4 0.2 0

0 0.2 0.4 0.6 0.8 1.0

0.0 0.2 0.4 0.6 0.8 1.0

15-25 15-55 15-60 15-60 15-60 15-60

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Fig. 2. Current density vs. molar fraction of chromic acid (A) and vs. anodizing voltage (B); and (C) logarithm of current density vs. voltage.

For all the voltages, the current density decreases linearly with the molar fraction of chromic acid xCr in the electrolyte (Fig. 2A, Table SI1). Elabar et al. reported anodization in chromic acid with two different contents of sulfuric acid (level of ppm) [44,45]. They concluded that anodizing in chromic acid with higher amount of sulfuric acid provides higher current density [45], which is in line with present results. The different sets of data-points in Fig. 2A, displaying the current density j as a function of the molar fraction of chromic acid xCr for different values of anodization voltage U, have been fitted with straight lines. The slope of the best fitting lines increases with voltage up to 45 V, and then drops rapidly. The values of slope

Fig. 3. Oxide growth rate vs. molar fraction of chromic acid (A) and vs. anodizing voltage (B); and (C) logarithm of oxide growth rate vs. voltage.

resulting from the fits in Fig. 2A have been plotted versus the anodization voltage U in Fig. 2B. Typically, during classical aluminum anodization in non-mixed electrolytes, including the cases of aqueous sulfuric acid [39] and chromic acid [46], the current density j increases exponentially with U [19]. However, in our mixed electrolyte we do not observe such a behavior, but rather, after achieving a maximum, the current density finally decreases with increasing voltage (Fig. 2B). Additionally, what is unusual, the logarithm of the current density is proportional to the

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Fig. 4. Correlation between oxide growth rate and current density.

squared voltage (Fig. 3B, Table SI2), whereas this relation is typically exponential [39,46]. The reason has to be found in the composition of the electrolyte. At acidic pH chromate anions often turn into dichromate anions, and those also are attracted by the anode, however their relatively large hydrodynamic radius causes low ionic mobility, and consequently low anion concentration at the electrode surface, under low voltage. The ion migration velocity V can be expressed as [43]: V ¼ uE ¼

ze E 6pha

ð3Þ

with the know meaning of the involved quantities. Hence, at higher voltages i.e. higher electrical fields E (at same distance from the electrode), relatively large anions are attracted more efficiently to the anode, and thus more dichromate anions are present on the anode surface. The Cr2O72 anions are known in anodizing industry as efficient sealers, improving corrosion resistance of anodic oxide coatings on technical purity aluminum, due to a more compact, less porous form of the grown anodic oxide. As a consequence, the current density exhibits a drop with voltage increasing in excess of 40 V, due to the attraction of greater quantity of passivating agents–chromates anions. It should be noted, that simultaneously quantity of the incorporated chromates is small enough to not influence significantly the electric properties of the oxide (according to Vrublevsky et al. anionic species incorporated into AAO for anodizing in tartaric acid change the values of the bandgap of the oxide [47]). The oxide growth rate d is strictly related to the current density and chemical composition of the electrolyte in consequence. Therefore, oxide growth rate is influence in the same way by chemical composition of the oxide (Fig. 3A, Table SI3) and the voltage (Fig. 3B–C, Table SI4). As could be expected, oxide growth rate and current density are in linear correlation (Fig. 4), what confirms that anodization is a typical faradic process (the volume of the obtained oxide is linearly related to the current density [32,44]). The influence of the relative electrolyte composition on the morphology of the grown anodic oxide has also been investigated, and representative SEM images of the different AAO surfaces are shown in Fig. 5. Despite anodization in sulfuric acid at 25 V (selfordering conditions; Fig. 5A), the arrangement is poorer than one could expected. It is probably due to the application of lower sulfuric acid concentration (1.0 M) than typically applied for the

fabrication of highly-ordered AAO [39]. It would be in line with the Pashchanka-Schneider model which correlates ordering not only with the voltage (for given electrolyte), but also with the pH, conductivity and viscosity of the electrolyte [48]. It is also in line with the data for oxalic acid, acquired by Shingubara et al. [49]. It this case, the lower the concentration of oxalic acid, the worse arrangement of the nanoporous arrays. According to other paper reported by Shingubara et al. [33], composition of the electrolyte (sulfuric/oxalic acid mixed electrolyte was researched) also influences the ordering of AAO. It is well-known that anodization in sulfuric acid, brings the best ordering at 25 V [39], while anodizing in chromic acid results in poor ordering of AAO [46]. According to Fig. 5, for chromic acid molar fraction of 0.2 or even 0.4 M, the ordering is satisfactory, however, further chromic acid molar fraction increase totally disorders the AAO arrays, what also could be explained by the Pashchanka-Schneider approach (totally different pH and conductivity of the electrolyte with high content of chromic acid and low content of sulfuric acid)[48]. Depending on the molar ratio of chromic oxide xCr, at fixed voltage U, differences in pore diameter DP and interpore distance DC appear. Generally, it is believed that DC is influenced mainly by the applied voltage, in linear manner, see Fig. 6B. However, the composition of the electrolyte also influences this parameter, as visible in Fig. 6A. It appears that, for all the applied voltages, the interpore distance DC increases exponentially with FC (Fig. 6A, Table SI5). For example, for the AAO formed at 25 V, for the limiting cases of pure acids, DC increases from 38.6  0.5 nm in 1.0 M sulfuric acid to 51.9  3.5 nm in 1.0 M chromic acid. Analogous finding was reported for aluminum anodizing in electrolytes with varied viscosity [32]. This means that in both limiting cases of pure acids the ionic mobility u of the electrolyte plays a major role. This point is clear from Eq. (1), where both electrolyte viscosity h and hydrodynamic radius a of the electrolyte ions are inversely proportional to the ionic mobility u. Additionally, different types of anions attracted on the anodized surface provide different distribution of the electric field lines, what further strengthens the above effect. At the same time, the linear relation between the interpore distance DC and the voltage U was maintained (Fig. 6B, Table SI6), but the slope of the best fitting lines is strongly influenced by the electrolyte composition described by xCr. On the other hand, there is no clear relation between pore diameter DP and xCr (Fig. 7A). Nevertheless, also in this case a linear relation between DP and voltage U, typical for anodizing [19], appears (Fig. 7B, Table SI7). The slope of the straight lines best fitting the pore diameter vs. anodization voltage DP(U) data-points is rather constant, and no significant influence of the electrolyte composition is noticed (Fig. 8). However, as mentioned above, the slope of lines fitting the interpore distance vs voltage DC(U) data-points is clearly affected: the higher the molar fraction of chromic acid xCr the higher the slope (Fig. 7B), i.e. the faster is the increase in interpore distance with the anodizing voltage. Up to now, it was generally accepted, that for mild anodization the proportional constant of the Dc(U) is rather independent from the applied electrolyte and equals 2.5 nm/V [50]. Generally, the influence of the electrolyte on the slope of Dc vs. U is commonly known only for hard anodization [50]. Even if in our work mild anodization is researched, the experimental results are in qualitative agreement with the dependency proposed by Li et al. for high fieldanodization [51]:

ld ¼

j DC ¼ 1:57 þ e1600 U

ð4Þ

where ld is the slope of the function DC(U), and j is the current density as usual. According to Eq. (4), the lower the j, the higher the

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Fig. 5. Top-view FE-SEM images of AAO formed by two-step anodization at 0  C and 25 V, in electrolytes with increasing molar fraction xCr of chromic acid: FC = 0 (A), 0.2 (B), 0.4 (C), 0.6 (D), 0.8 (E) and 1.0 (F).

slope ld. In our work, for the highest values of molar fraction of chromic acid xCr, the lowest values of j were recorded, and the quantitative analysis of the FE-SEM images of the formed oxides allowed to find higher values of DC and, as a result, higher slopes ld of the fitting lines. Another morphological feature influenced by the electrolyte composition is the number of pores appearing in the unit surface area, known as pore density n, which in the case of defect-free close-packed hexagonal array of pores is strictly related to the interpore distance Dc [19,52]:

estimation of n with the use of Dc. It is found here that the higher the chromic acid molar fraction xCr FC the lower the pore density n (Fig. 9A), with linear trend (Table SI8). The linear decrease of n with xCr is linked with the exponential increase of the DC with xCr (compare to Fig. 6A). The pore density n decreases with the inversed square of the voltage U, what is in agreement with Eq. (5) (DC is a linear function of U) and literature data [19]. Nevertheless, the linear coefficient varies according to the electrolyte composition what is expected due to the influence of xCr on DC (Fig. 6A).

2  1014 n ¼ pffiffiffi 2 3  DC

4. Conclusions

ð5Þ

The obtained relations are the effect of the influence of the operating conditions on the interpore distance, due to the

Anodization of aluminum in electrolytes composed of various amount of sulfuric and chromic acids has shown that changes in the electrolyte composition may significantly influence the growth

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Fig. 6. Interpore distance vs. molar fraction of chromic acid (A), and anodizing voltage (B).

Fig. 7. Pore diameter vs. molar fraction of chromic acid (A), and anodizing voltage (B).

of nanoporous oxide. The results obtained allow to draw the following conclusions for the AAO formation and growth:  The chemical composition of the anodization electrolyte in mixed sulfuric acid–chromic acid systems allows to tune the ionic mobility of the charge carriers.  Due to the relatively low ionic mobility of chromate anions, the current density decreases with the increase in molar fraction of chromic acid in the electrolyte.  For all the electrolyte compositions, the current density increases with voltage up to 40 V and then decreases, what is probably caused by more efficient attraction of chromates onto metal/growing oxide interface and consequently formation of more insulating (thicker) barrier layer.  The growth rate is a linear function of the current density, thus it behaves accordingly: it decreases linearly with the increase in the molar fraction of chromic acid and its logarithm is a square function of the anodizing voltage.  For all the electrolytes, the interpore distance increases linearly with the voltage, although for all the voltages the interpore distance increases exponentially with the molar fraction of chromic acid.

Acknowledgements W.J. Ste˛pniowski cordially acknowledges financial support from Polish Ministry of Science and Upper Education (Scholarship for

Fig. 8. Slope of the lines best fitting the sets of data-points of pore diameter vs. voltage and interpore distance vs. voltage, as a function of the molar fraction of chromic acid.

Young, Outstanding Researchers 2015–2018, agreement no. 0432/ E-410/STYP/10/2015). M. Norek acknowledges the financial support from the National Science Centre—Poland (Decision number: DEC-2012/07/D/ST8/02718).

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Fig. 9. Pore density vs. molar fraction of chromic acid (A) and anodizing voltage (B).

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