Observation of Pore Growth and Self-Organization in Anodic Alumina

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The Observation of Pore Growth and Self-Organization in Anodic Alumina by Time Resolved X-ray Scattering Nikolay A Vinogradov, Gary Harlow, Francesco Carlà, Jonas Evertsson, Lisa Rullik, Weronica Linpé, Roberto Felici, and Edvin Lundgren ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00303 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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ACS Applied Nano Materials

The Observation of Pore Growth and Self-Organization in Anodic Alumina by Time Resolved X-ray Scattering Nikolay A. Vinogradov1,2, Gary S. Harlow3*, Francesco Carlà1, Jonas Evertsson3, Lisa Rullik3, Weronica Linpé3, Roberto Felici1,4, Edvin Lundgren3

1

ESRF – The European Synchrotron, 71 Avenue des Martyrs, 38000 Grenoble, France

2

MAX IV Laboratory, SE-22594, Lund, Sweden

3

Division of Synchrotron Radiation Research, Lund University, SE-22100 Lund, Sweden

4

SPIN-CNR, c/o DICII-University of Rome Tor Vergata, Via del Politecnico 1, I-00133 Roma, Italy

*Corresponding author. Email: [email protected]

Abstract

The anodic oxidation of metals such as aluminium and titanium can lead to the development of self-ordering pores. These pores make excellent templates for a range of nanoscale objects with many applications in nanoscience. Theoretical studies on pore formation have proposed several models for the establishment, growth and ordering of these pores, however experimental verification has mostly been limited to ex-situ measurements. Here we show that the lateral and vertical pore structure can be probed in-situ with high precision using Grazing Transmission Xray Scattering. By making use of the high flux available at modern synchrotrons and fitting only the difference between scattering patterns we show the near real-time evolution of the pore’s arrangement. We observe no dependence on the substrate crystallographic orientation for domain size or pore separation. We do however observe an anisotropy in the oxide growth rate for the different substrate surfaces. This experimental approach can be applied to the study of a large variety of electrochemically produced materials such as magnetic nanowires, novel solar cell designs and catalysts. 1 ACS Paragon Plus Environment

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Keywords: aluminium anodization, nano pore, self-organization, GTSAXS, small-angle scattering, porous membrane, X-ray, electrochemistry Introduction Porous anodic alumina (PAA) is formed by the electrochemical anodization of aluminium in various electrolytes 1. Certain electrolytes and voltages have been shown to cause a self-ordering of the pores into a hexagonal pattern 2,3. Pore diameters can be tuned in a range of 10 to 400 nm whereas pore heights can be up to tens of micrometres 4. Remarkably the selfordering is coherent over micrometre sized domains (known as coherent domains) each with the same structural parameters (pore diameter and pore separation) but a different orientation. After long periods of anodization the orientational alignment of these coherent domains also increases5,6. The range of possible aspect ratios of PAA has attracted much interest and has found potential applications in nanoscale fabrication 7, magnetic storage 8, catalytic membranes 9, batteries 10 and photocatalysis/solar cells 11 as well as being used industrially as an adhesive base for decorative colouring processes. Despite the importance of PAA, the reason for the selforganization of the pores is not yet fully understood where the importance of both stress and dissolution for self-organization is not completely clear. It is known that during electrochemical anodization metal ions from the oxidation of the substrate, and oxygen ions from water migrate through the oxide layer due to the high electric field (~ 1 V nm-1) created by the applied voltage

12–14

. Some of this ionic current results in the

field-assisted ejection of Al3+ cations into the solution but most (60 – 90 %) results in oxide growth 15. One theory of pore formation is that a morphological instability begins to

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Figure 1| Schematic of the experiment. a, Illustration of the electrochemical cell, Al3+ ions migrate through the anodic alumina oxide (AAO) and are ejected into solution. O2- ions at the anode (working electrode) are incorporated into the new oxide whereas H2 bubbles formed at the cathode (counter electrode) flow out of the cell. Polished hat shaped aluminium working electrodes were used. b, Illustration of the GTSAXS geometry with a vertical beam stop shown. Qz probes the out-of-plane structure and Qy the in-plane structure.

develop in the early stages of oxide growth that leads to pore formation

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. The competition

between oxide growth and dissolution processes then leads to pore formation. It has been suggested that an important driving force for pore formation and ordering is mechanical stress which is induced by the incorporation of electrolyte species into the oxide matrix 16,17. The exact mechanism is still under debate and these models fail to account for long-range ordering or the 3 ACS Paragon Plus Environment


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observation of differences in the orientational order when different aluminium single crystal faces are anodized 18,19. The sensitivity of in-plane orientational order to crystallographic orientation is surprising considering that the oxide layer is amorphous and the pore separation is several orders of magnitude greater than the lattice constant of the Al substrate. The average size of defect free domains are found to follow the trend (110) < (111) < (100), after long anodization times

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. It

has been suggested that the crystallographic orientation of spikes that form at the metal/oxide interface due to the hemisphere like pore base is impacted by a difference in oxidation rates of different crystallographic planes.19 These spikes then transfer their orientational correlation to the pores as the oxide growth deepens into the metal substrate with crystallographic planes oriented closer to {111} showing the slowest rate of film growth. Experimental validation of these models of pore growth has, however, been mostly limited to ex-situ measurements separated by large time intervals. Typical techniques used to examine nanoscale structures are atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). These post-factum methods often require additional sample processing stages such as cleavage or metallic sputtering. Ideally measurements of pore geometry and growth should be made such that the process of interest can be observed in real time. Previous in situ measurements have used transmission small-angle X-ray scattering (SAXS)

19–21

, however the need for the beam to penetrate through

the substrate imposes some limitations on sample thickness. Further limitations of the transmission SAXS approach are a limited temporal resolution for changes in the oxide perpendicular to substrate, which can only be measured by tilting the sample (a rocking scan). Here we report in-situ grazing-incidence transmission small-angle X-ray scattering (GTSAXS) 4 ACS Paragon Plus Environment

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measurements during the anodization of aluminium samples with different crystallographic orientations. Despite its simplicity, the GTSAXS technique was only introduced recently 22 and our measurements demonstrate how this technique can be used for fast in-situ investigations of nanoscale electrochemical systems such as PAA. The X-ray beam in the grazing transmission geometry (illustrated in Fig. 1) only needs to penetrate a small amount of the substrate (compared to a standard transmission geometry), therefore a stronger scattering signal can be detected. At 21.5 keV the attenuation length for aluminium is ~1.4 mm whereas our samples are 7 mm thick. Analysis of GTSAXS data is also considerably simpler than that of the reflected scattering normally measured in grazing incidence small angle X-ray scattering (GISAXS) because the incident angle of the radiation is much larger than the critical for total internal reflection. Therefore only the ‘Born Approximation’ and not the full ‘Distorted Born Wave Approximation’ is needed to model the data

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. In the case of

GTSAXS the length of the pores are oriented nearly perpendicular to the incident beam, therefore additional information about their out-of-plane structure (along the Qz reciprocal space direction) is available without rocking the sample. For the measurements reported here, the scattering was nearly homogenous with rotation (a mosaic of many rotated domains is simultaneously sampled) suggesting no long-range correlational order between the different domains over the anodization time investigated. Aluminium single crystals with polished (100) and (111) faces and a polycrystalline sample have been anodized at 40 V (constant voltage) in 0.3 M oxalic acid at ~ 4 °C using a 2step procedure known to give well-ordered uniform pores23. A schematic of the electrochemical cell used is shown in Fig. 1a. The sample and cell were tilted at glancing angles approximately 5 times larger than the critical angle (αc) of aluminium in water, such that the reflected and 5 ACS Paragon Plus Environment


transmitted scattering are well separated, and the diffracted signal is at a vertical height on the detector close to the direct beam. Fig. 2c shows three frames during the anodization of the Al (100) sample, where the frames are cropped to show only the right-hand side of the symmetrical pattern about the beam stop. 1st anodization: Pore self-organization During the first anodization, for the first 60 s the scattering is confined to a horizontal streak (due to in-plane roughness) with no features along Qz. This corresponds to initial stages of pore formation (Fig 2a-I) which is characterized by a dip in the measured current (Fig. 2b.) due growth of the barrier layer

12,24,25

. At longer times, peaks along Qz = 0 nm-1 start to appear, a

signature of the quasi-hexagonal pattern of the pores. The first peak appears at Qy ~ 0.07 nm-1 (Fig. 2a-II). The entire sequence can be seen in the movie M1. The scattered intensity is a product of the average pore form factor F(Q) and a lattice structure factor S(Q). For a hexagonally packed structure S(Q) has maxima at = 4√ℎ + ℎ + /(√3) where ‘a’ is the unit cell dimension (i.e. pore separation) and h,k are the Miller indices. The position of the first few peaks are indicated in Fig. 2c-III (although somewhat difficult to resolve on the image by eye due to the broadening) are in good agreement with those of a hexagonal lattice but the higher order peaks are slightly displaced because the PAA is only quasi-hexagonally ordered (i.e. it is a paracrystal26). With sufficient time new oxide forms deeper into the metal with an increased pore order compared to that from earlier in the anodization process, with the old oxide at the oxide/electrolyte interface showing the least order (Fig. 2a-III). The scattering in each GTSAXS image is the sum of the entire oxide layer that is in the path of the X-ray beam. The oxide grows deeper into the metal with time. To understand how the 6 ACS Paragon Plus Environment

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Figure 2| Pore ordering during 1st Anodization at 40 V. a, Illustration of the pore alignment process, Ipore initiation, II – Pore alignment, III – Growth of ordered pores b, Current-time transient during the anodization of a polycrystalline Al sample under similar conditions. c, GTSAXS detector images recorded after 0 s (c-I), 250 s (c-II), and 7200 s (c-III) of anodization for Al (100) (See M1), images are normalised to an ionisation chamber monitor before the sample. d, The evolution of average pore separation as a function of time obtained from fits to data along Qz = 0. e, The domain size as a function of time for the three samples investigated, the solid line is a power-law fit to the data as described in the text.



structure of the new oxide evolves with time it is necessary to ascertain changes in the scattered intensity. To achieve this goal, we sum the intensity of several images and then subtracted the scaled intensity of the image immediately before the averaged images (see supporting information). This method is only applicable along the GTSAXS component parallel to the sample surface (L = 0) where the scattering sums incoherently. Here we make the assumption that as the new oxide forms the old oxide remains unmodified, this is reasonable approximation given how the oxide grows at low temperatures12,13. Furthermore, it is not expected that the scattering from the old oxide will be significantly affected by any viscous flow of the oxide27 as this mostly occurs in the new oxide close to the pore bottom being measured. In-plane structural parameters can then be determined by fitting the data along the line at Qz = 0. A model based on previous ex-situ models of hexagonal nanostructures

28,29

was used to fit data and obtain values

for the pore separation and coherent domain length (see supporting text). The evolution of the pore separation and the coherent domain length for all 3 samples is shown in Fig. 2d and Fig. 2e respectively. The average pore separation at the end of the 2-hour anodization was around 100 nm whereas the coherent domain length was around 800 nm. These are in good agreement with our ex-situ measurements (see supporting information). The time at which the largest increase in pore separation takes place corresponds well with the rise in current after pore formation occurs (Fig. 2b), suggesting considerable activity of the pores during this phase. Fig. 3 shows how the pore separation evolves in time for different anodizing voltages. For higher voltages (e.g. 50 V and 60 V) the etching conditions at the pore bottoms are strongly time-dependant and this leads a decline in measured current over time, accompanied by worse ordering of the pores 30. Since we only anodize for a relatively short time


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Figure 3| Average pore separation at different voltages. The evolution of average pore separation as a function of time obtained from fits to data along Qz = 0. The data was measured during the anodization of polycrystalline aluminium in 0.3 M Oxalic acid. Solid lines show fits to the data using the model described in the text, parameters are given in supporting information. The increased noise in the 60 V line is an experimental artefact induced by an attenuator change and the first diffraction maxima being closer to the beam stop and is not indicative of the pore structure.

period this does not significantly affect our measurements. The solid lines in Fig. 3 are a fit to the data using a sigmoidal function (see supporting information). This is the first time we are aware of that the early dynamics of pore separation have been measured in-situ or investigated at different voltages. This time resolved data on pore ordering should be useful in testing the various theories that currently exist to explain the self-ordering phenomena.



Interestingly we found no significant difference between the different crystal faces (Fig 2e) or voltages (Fig. S1) with respect to the average domain size. In contrast, reported ex-situ measurements of anodization over much longer time scales show a difference in the long-range ordering and density of defects depending on crystallographic orientation6,18. However, the order parameter we describe here is not equivalent, and such changes only become apparent after much longer anodization times, where a longer-range order has been reported19. The coherent domain length (Fig. 2e) is found to follow a power law (L = Btn where B is a function of temperature), for our measurements, a value of n = 0.25 ± 0.04 best fit the complete data, as indicated by the black line. This implies that the domain area grows with a value n=0.5 and is in agreement with earlier studies that found similar exponents20,31. However, both these previous reports on domain growth were however were made at much longer time scales, with few data points over the early stages of anodization that is investigated in this report. It is also worth mentioning the parameter measured here is an average and assumes the domains are approximately circular. This kind of power law behaviour is similar to that of grain growth in metals, albeit with a different exponent, where the driving force is a reduction in the boundary energy

31,32

. It has been suggested that the growth of these domains is similar to Ostwald

ripening20. Some pores close to the domain boundaries stop growing and the adjacent pores split into two channels at positions that merge the domains. This is of course the reason for why longer anodization times are often used, to increase the size of coherent domains, it is clear from Fig. 2d and Fig. 3 that the separation between the pores themselves plateaus rapidly. 2nd anodization: Aligned pore growth


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After chemical etching of the oxide layer which was formed in the first anodization process, a second anodization process was investigated using GTSAXS. These results are summarized in

Figure 4| Oxide growth during 2nd anodization stage. a, GTSAXS images recorded after I - 0 s, II – 250 s, and III - 7200 s of the 2nd anodization for Al (100) at 40 V in 0.3 M Oxalic acid, images are normalised to an ionisation chamber monitor before the sample b, Oscillations in the intensity along Qz at the first Bragg peak (Qy ~ 0.07 nm) for the Al (100) sample at various times. c, The calculated pore length from the period of the oscillations in Qz for the three samples. The lines show the linear fits used to determine growth rates.

Fig. 4a for the Al (100) sample where the entire time evolution can be viewed in movie M2. Due to a pre-patterning of the surface in the first anodization stage, a streak of intensity along Qz at 11 ACS Paragon Plus Environment


the (1 0) Bragg peak position (Qy ~ 0.07 nm) is already visible. Since the self-ordering process has considerably slowed during the first-step we observe very little change in the parameters discussed in the previous section, over a 2-hour anodization (see supporting information). In the early stages of oxide growth oscillations along the Qz direction are visible on the detector images (see Fig. 4b). These are characteristic of the scattering from 1-dimensional objects (i.e. the cylindrical pores), and are present for the second anodization because the pores are more uniform in shape and height compared to the first anodization. The pore height (H) can be determined from the relation = 2/∆ where ∆ is the distance between adjacent oscillations. Fits of a cylindrical scattering form factor to the measured intensity along Qz are shown in Fig. S2 at various time intervals (see supporting information). Fig. 4c shows the evolution of the calculated pore heights during the early stages of the 2nd anodization for each of the samples investigated. There are clear differences between the three aluminium faces, the fastest growth occurs with the (100) sample which was found to grow at 1.6 nm/s (from the linear fit indicated with the red line). The slowest rate of growth in our samples was found for (111) which grew at 1.2 nm/s. The polycrystalline sample had a rate of oxide growth of 1.4 nm/s. It is already known in the very early stages of growth (where there is a large change in current) that the pore growth is not linear with time and much slower

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therefore the lines do not pass through the origin. Fig. S7

shows a plot of the charge consumed against pore height which does indeed pass through the origin within the error of the experiment. The relative simplicity with which growth rates, pore separation and domain sizes can be calculated from the in-situ experiments presented here, shows why GTSAXS can be an important technique for understanding the growth of nanoscale materials such as PAA and their functionalization. Conclusions


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The technical feasibility of in-situ GTSAXS during the electrochemical growth of porous oxides, such as PAA, is demonstrated. We have introduced a difference GTSAXS technique that allows changes in the pore separation and average coherent domain size to be followed throughout the anodization process and compared for different crystal faces as these parameters evolve. Previous SAXS measurements have only considered how the overall average throughout the oxide layer evolves in time. We have shown how pore separation and average domain lengths for PAA evolve in time during the early stages of anodization for three different crystal faces and as well as with different voltages. Our measurements gave similar results for all the crystallographic orientations investigated, despite a difference in the oxide growth rates. This suggests that the positional correlation of the pores is independent of the height of the oxide layer above. We find in agreement with previous studies

31

that domain growth follows a power law like that of grain

growth in metals, albeit with a different exponent, where there is a tendency to minimise the boundary energy. It has also been shown how the growth rates of the aluminium oxide (i.e. pore heights) for the various crystallographic orientations can be measured in-situ with GTSAXS. The growth rate was found to be greatest for the (100) face. Furthermore, our approach demonstrates how GTSAXS can be used for real time in-situ measurements of this important class of materials, opening the door to further in-situ studies in harsh conditions during the electrochemical production of nanoscale solar cells, catalysts and membranes. Experimental Procedures



The aluminum samples were Al(100) and Al(111) single crystals and polycrystalline Al (from Goodfellow, 99.995 % purity) extruded from an Al rod, machined in a hat-shape with the diameter of 7mm on the polished side. The samples were mechanically polished by Surface Preparation Laboratory, Netherlands (SPL) with final polishing step ≤ 0.03 µm. The alignment of the crystallographic faces was to ~0.1 o. These were anodized in a home-built polyether ether ketone (PEEK) electrochemical cell under potentiostatic conditions (Vanod = 40 V), at 3-5 oC and constant electrolyte flow, the bath temperature was -7 oC but the ambient temperature increased the temperature of the electrolyte before it reached the cell. A 0.3 M oxalic acid (H2C2O4) +5 wt.% ethanol (C2H5OH) solution in ultra-pure water was used as an electrolyte. The presence of ethanol was used to prevent the electrolyte freezing in the bath. For each sample, we have employed the well-known two-step anodization process2, where after an initial anodization the oxide layer is stripped in a mixture of chromic and phosphoric acid – a subsequent anodization then produces well-ordered and aligned pores. Constant pumping of the electrolyte through the cell also allows for controlling the temperature of the electrolyte in the electrochemical cell, and for removing the reaction heat from the small volume of the cell.

For each sample the first anodization step lasted ~3 hours, the second anodization step was done for a shorter time, around 2 hours, due to fewer changes in the scattering pattern at higher anodization times. An energy of 21.5 keV photons (λ = 0.05777 nm) at the ID03 (ESRF) beamline

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was used for the experiment. A Maxipix-X5 detector placed 2.2 m away from the

sample, in the path of the direct X-ray beam, and used for capturing the scattering pattern. The detector dimensions are 256(v) x 1296(h) pixels, where each pixel has a square shape and dimensions of 55 x 55 µm. About 3 mm in front of the detector a 15(v) x 0.5(h) mm tantalum beam stop was mounted in order to prevent the detector oversaturation by the direct and reflected 14 ACS Paragon Plus Environment

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beams. A framerate of up to 1 Hz was achievable but reduced to 0.1 Hz to minimize the effect of beam damage. The set up with such parameters allows the detection of the momentum transfer from Qmin = 0.0136 ± 0.0027 nm-1 to Qmax = 1.765 ± 0.0027 nm-1 with the resolution in the reciprocal space of 2.72x10-3 nm-1. This allows for observing the structural features from the objects with typical sizes in the range from 462 nm to 3.55 nm, respectively. The diffraction pattern was corrected for refraction (which was in any case small due to the high incidence angle) by assigning the center pixel Qy,z = 0 from the symmetry of the diffraction pattern after anodization. References (1)

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(29) Engel, M.; Stühn, B.; Schneider, J. J.; Cornelius, T.; Naumann, M. Small-Angle X-Ray Scattering (SAXS) off Parallel, Cylindrical, Well-Defined Nanopores: From Random Pore Distribution to Highly Ordered Samples. Appl. Phys. A 2009, 97, 99–108. (30) Jessensky, O.; Müller, F.; Gösele, U. Self-Organized Formation of Hexagonal Pore Structures in Anodic Alumina. J. Electrochem. Soc. 1998, 145, 3735–3740. (31) Li, F.; Zhang, L.; Metzger, R. M. On the Growth of Highly Ordered Pores in Anodized Aluminum Oxide. Chem. Mater. 1998, 10, 2470–2480. (32) Hillert, M. On the Theory of Normal and Abnormal Grain Growth. Acta Metall. 1965, 13, 227–238. (33) Kim, M.; Kim, H.; Bae, C.; Lee, J.; Yoo, H.; Moreno, J. M. M.; Shin, H. Initial Self-Ordering of Porous Anodic Alumina: Transition from Polydispersity to Monodispersity. J. Phys. Chem. C 2014, 118, 26789–26795. (34) Ferrer, S.; Comin, F. Surface Diffraction Beamline at ESRF. Rev. Sci. Instrum. 1995, 66, 674–1676. Acknowledgments The experiments were performed on the ID03 beamline at the European Synchrotron Radiation Facility (ESRF). This work was financially supported by the Foundation for Strategic Research (SSF) Project No. RMA11-0090, and the Swedish Research Council by the Röntgen-Ångströom Cluster "In-situ High Energy X-ray Diffraction from Electrochemical Interfaces (HEXCHEM)" (Project No.). Author Contributions N.A.V, F.C and R.F conceived and designed the experiments, G.S.H and N.A.V analysed the data and wrote the manuscript. All authors preformed the X-ray experiments and discussed the results. Supporting Information

Supporting Text Figures S1 to S4 Movies M1 and M2



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Observation of Pore Growth and Self-Organization in Anodic Alumina

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