AFM, SEM and TEM Studies on Porous Anodic Alumina
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- Zhu, Y.Y., Ding, G.Q., Ding, J.N. et al. Nanoscale Res Lett (2010) 5: 725. doi:10.1007/s11671-010-9538-9
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Porous anodic alumina (PAA) has been intensively studied in past decade due to its applications for fabricating nanostructured materials. Since PAA’s pore diameter, thickness and shape vary too much, a systematical study on the methods of morphology characterization is meaningful and essential for its proper development and utilization. In this paper, we present detailed AFM, SEM and TEM studies on PAA and its evolvements with abundant microstructures, and discuss the advantages and disadvantages of each method. The sample preparation, testing skills and morphology analysis are discussed, especially on the differentiation during characterizing complex cross-sections and ultrasmall nanopores. The versatility of PAAs is also demonstrated by the diversity of PAAs’ microstructure.
Porous anodic alumina (PAA) has been discovered and applied in many industrial areas for more than a century. Due to its highly ordered hexagonal nanopore array and its wide applications for fabricating various nanostructured materials [1–3], the research on PAA has been very active during the past decade. The current PAA studies focus on the improvement of regulation, better controlling on pore diameter, interpore distance and thickness, high-speed growth, new types of pore structures [4–7], as well as its new applications [8, 9]. These studies were mainly characterized by atomic force microscopy (AFM) [10, 11], field emission scanning electron microscopy (FESEM) [12, 13] and transmission electron microscopy (TEM) [14, 15], and each characterizing method has its own advantages and disadvantages.
Generally, AFM can be only used to observe the PAA surface, and it has the advantage of no requirement on the conductivity of samples, and it also has the capability for characterizing PAAs with smaller nanopores. FESEM is widely used to observe the surface and cross-sections, and measure the thickness of different samples. We can use FESEM to fast relocate and obverse the desired sample area within several tens of nanometers to hundreds of micrometers, but the samples must have good electrical conductivity. Poor electrical conductivity will lead to blurry photos, unacceptable brightness variation and even characterization failure. Sometimes, in order to obtain better electrical conductivity, the conductive layers sputtered on samples may be so thick or the grain size is so large that the sputtered grains influence and even cover the true microstructure of PAA. TEM can be used to observe the cross-sections by slicing and thinning the sample, and observe the surface morphology by controlling the thickness of the sample within 100 nm. TEM also can be used to characterize PAAs’ composition and crystalline structure. But the pretreatments on samples are complicated and time-consuming, so AFM and FESEM are the most commonly used methods for observing PAAs’ microstructure.
However, PAA and its evolvements vary too much in microstructures, as well as in its application requirements as nanoscale templates. The pore diameter (10–500 nm), thickness (50–200 um), interpore distance (20–1,000 nm) and other structure parameters of PAAs can be continuously adjusted over a wide range, and the nanopore shapes can be circular, diamond/diamond-triangle  and square  by the aid of mold-pressing or lithography techniques. And the ordered nanopore array can transform into nanowires, nanotips, nanorods, nanosteps by post-treatments or tuning the anodizing process [18–20]. All of these different characteristics of PAA surfaces and sections require different techniques and skills of AFM, FESEM and TEM, but a systematical study on these characterizations has not been reported so far.
We already have a lot of experiences and good results on fabricating various PAA templates and their applications for fabricating nanostructure materials [20–24]. During our past works, we have accumulated some experiences and skills of characterizing different PAAs. In this paper, we have detailed AFM, FESEM and TEM studies on PAAs and their evolvements with different microstructures, and discuss the advantages and disadvantages of these methods. The structures of PAAs in this research include the regular PAA structures with different pore diameters, nanowires, nanotips and microstep-nanopore hierarchical structure. In particular, we discuss the characterization of PAAs’ unique fracture behaviors and complicated multilayer structures, PAAs with ultrasmall nanopores and the PAAs’ electrical conductivity. These works provide the techniques and skills for observing and identifying the special structures of all kinds of PAAs to the researchers.
High purity (99.999%) aluminum foils with the thickness of 200 μm were employed for fabricating PAAs. The as-rolled foils were immersed in acetone for few minutes and then washed in deionized water without further electrochemical polishing or thermal treatment. The regular PAA templates with different diameters and thickness were obtained in different electrolytes at different applied voltages and anodizing durations. PAAs with ultrasmall nanopores (5–20 nm) were prepared in 20 wt% sulfuric acid at 1–15 or 1–10 V in 0.3 M oxalic acid. Medium-diameter (40 nm) PAAs were prepared in oxalic acid with an operation voltage of 40 V. Large-diameter (>100 nm) PAAs were achieved in the phosphoric acid under higher voltage. The alumina nanowires were obtained by dissolving partial PAA wall in 5 wt% H3PO4 under 30°C for a period of time. The alumina nanotips were prepared at 20°C in a mixed electrolyte. PAAs with multilayer structures were prepared by two-step or multi-step anodization process under different voltages. All the samples were washed in deionized water twice by the aid of water-bath ultrasonic, then dried before test. The detailed experimental information of each sample can be found in the figure caption.
The AFM topography examinations were carried out on a Nanoman VS AFM system under tapping mode with RTESP probe from U.S.A.Veeco Instruments. The cantilever of RTESP probe has straight beam structure with a radius 8-nm pyramid tip. A FEI SIRION 200 FESEM (FEI, U.S.A., resolving power of 3.0 nm at 5 kV) and JEM-2010 TEM (OXFORD, U.K., point resolution of 0.25 nm) were used for morphological characterization of PAA.
Results and Discussion
FESEM is the most commonly used method for observing PAAs’ microstructure, since FESEM can not only observe the surface, but also the transverse, tilt and cross-sections, as well as analyze the elemental composition by the energy dispersive X-ray analysis. Figure 3a, 3b is the typical surface and cross-sectional FESEM images of typical PAAs, respectively. The view field of FESEM is so wide that we can relocate the targets quickly, and that a systematically study on PAA microstructure can be accomplished in short time. Another advantage of FESEM is its low requirements on sample preparation, since the micro-scale roughness and nano-scale height fluctuations can be easily overcome by adjusting the distance between sample and probe and focusing process.
Complicated structures of PAA evolvements (modified, post-treated or tuned PAAs) can also be well characterized by FESEM due to previously mentioned advantages, as shown in Fig. 3c–3e. Figure 3c reveals that a lot of nanoscale protruding objects on the surface and nanoparticles in the nanopores of the PAA, which is fabricated in oxalic acid and treated in hot water. Figure 3d, 3e is the cross-sectional and surface images of nanowires standing on PAA, respectively, which is obtained by anodizing aluminum under higher temperature of 50–70°C in 0.3 M oxalic acid. The PAA nanopores transfer into nanowires due to partial dissolution of the pore wall under higher temperature, and the nanowires are so long that they collapse into some several-micron protruding bunches on the surface. AFM can detect the nanowires in a small area, such as 1 μm2, as shown in Fig. 2b, but it is very hard to characterize the nanowires and micro-scale bunches at the same time.
The surface of general PAA samples can be easily characterized by AFM and FESEM. The element distribution and crystalline structure can be done with TEM.
The fracture characterization of PAA should be done with FESEM since it is most convenient and powerful to observe the transverse, tilt and cross-sections, the surface by bending the sample directly or scratching the surface with a diamond knife.
Ultrasmall PAAs with the pore diameter 5–20 nm should be characterized by AFM and TEM. AFM is better than TEM for this characterization.
PAA is very versatile since it has ordered hexagonal nanopore array and many evolvements that are obtained by post-treatments and tuning the fabricating process.
This work was supported by New Century Excellent Talents (NCET-04-0515), Qing Lan Project (2008-04), Key Programs for Science and Technology Development of Jiangsu (BE20080030), Changzhou Science and Technology Platform (CM2008301) and Key Laboratory of Material Tribology of Jiangsu (KJSMCX0902).
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