Morphology of TiO2 nanotubes revealed through electron tomography

Micron 95 (2017) 35–41

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Morphology of TiO2 nanotubes revealed through electron tomography b ´ M. Andrzejczuk a,∗ , A. Roguska b , M. Pisarek b , M. Hołdynski , M. Lewandowska a , a K.J. Kurzydłowski a b

Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 7 September 2016 Received in revised form 16 December 2016 Accepted 16 December 2016 Available online 27 January 2017

a b s t r a c t In this work, scanning-transmission electron microscopy (STEM) tomography was successfully applied to characterize the three-dimensional structure of titanium oxide nanotubes prepared by the electrochemical anodization of the Ti substrate. The results provided detailed information about the morphology of nanotubes as well as insight into their growth. The segmentation of reconstructed images made it possible to estimate the surface area and volume of the nanotubes. The highest specific surface area was obtained for the lowest anodization voltage of 10 V, and corresponds closely to that obtained using the porosimetry technique. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction The electrochemical formation of tunable nanoscale oxide layers on metallic surfaces has recently captured a lot of attention in materials research. Such materials possess a unique morphology, which can be optimized in a simple way by changing the parameters of the fabrication process, such as anodic voltage and electrolyte composition. The precise control of porosity at the nanometer scale makes it possible to fabricate new materials with special properties, which can be used in many fields of applications, such as photocatalysis, heterogeneous catalysis, lithography, optoelectronics, biomedicine (implants), protective and self-cleaning coatings, biosensors, templates for fabricating nanorods, etc. (Roy et al., 2011; Zhou et al., 2015; Pisarek et al., 2013; Nischk et al., 2014). Highly ordered porous structures are mostly studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which are very suitable techniques for nanoscale characterization (Roguska et al., 2011; Pisarek et al., 2013). However, these techniques are very often insufficient for describing complex, three-dimensional nanostructures. The most popular SEM plane-views or cross-sections provide only limited information about 3D morphology. The TEM observations of thin lamellas can enhance this analysis, but provide only limited information on the internal structure of nanotubes, and any quantification of these results is highly questionable.

∗ Corresponding author at: Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland. E-mail address: [email protected] (M. Andrzejczuk). http://dx.doi.org/10.1016/j.micron.2016.12.008 0968-4328/© 2017 Elsevier Ltd. All rights reserved.

Currently, the most popular technique in 3D imaging is Xray tomography, which features a lower resolution exceeding one micron (Salvo et al., 2010). Better resolution, in the range of tens of nanometers, can be achieved using a focused ion beam/scanning electron microscope (FIB/SEM) (Holzer et al., 2004). Atomic-level resolution in 3D imaging can be obtained by the application of the atom probe tomography technique (Oveisi et al., 2013). However, this technique is primarily designed for 3D chemical imaging. More information about the geometry of nano-sized structures can be obtained using the electron tomography technique (Midgley and Dunin-Borkowski, 2009). This method has widely been used in biology (Marco et al., 2004), and is now increasingly being applied in materials sciences as well (Wu et al., 2013; Hindson et al., 2011). Electron tomography is particularly attractive for imaging nano-sized features such as titania nanotubes, where a better understanding of their growth process can be obtained from threedimensional structure reconstruction. This technique provides a resolution of about one nanometer, and three-dimensional imaging of nano-sized objects in multiphase systems (Zeˇcevic´ et al., 2013). However, a more accurate analysis is achieved only when the “missing wedge” problem is eliminated. This can be done by using a holder in which a needleshaped specimen is rotated about a cylindrical axis (Yaguchi et al., 2008). Needle-shaped specimens are usually prepared by FIB, and so special care must be taken to avoid the FIB milling damage which typically occurs (Andrzejczuk et al., 2010). Electron tomography in materials science is used to visualize the complex, three-dimensional structure of nanoparticles, nanowires or nanoporous materials (Dennenwaldt et al., 2016; HernándezGarrido et al., 2014; Lim et al., 2013; Biermans et al., 2010).

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It provides precise information on the morphology of heterogeneous nanomaterials, which are composed of different phases. The data obtained can be used as input to calculate and model the suspected properties of the nanomaterials (Perassi et al., 2010). The HAADF-STEM (high angle annular dark field) and BF-STEM (bright field) modes can be used to collect the tilt series images required for tomography reconstruction. HAADF-STEM projection images whose intensity is proportional to mean atomic number and specimen thickness fully satisfy the requirements of electron tomography (Weyland and Midgley, 2004). In this mode, diffraction contrast is minimized and only those electrons scattered at high angles are collected (Midgley and Weyland, 2003). In BF-STEM images, the presence of diffraction contrast could lead to artifacts in 3D reconstruction, and so this mode is only applied to poorly crystallized materials with a homogenous density of particles (Roiban et al., 2012). In this work, we used HAADF-STEM electron tomography to characterize titanium oxide nanotubes obtained under different anodization conditions. 3D visualization of the structure was used to investigate their morphology and the growth process during metal oxidation. Finally, a consistent analysis of the morphology of the nanotubes was used to determine the correlation with the Brunauer, Emmett and Teller (BET) measurements of surface area. 2. Experimental 2.1. Synthesis of nanotubes The TiO2 nanotubes examined were fabricated in an anodization process of titanium. A Ti foil of 0.25 mm thickness (99.5% purity, Alfa Aesar) was used as the substrate. Samples of 1 cm2 were cleaned ultrasonically with acetone and ethanol, rinsed with deionized (DI) water, and dried in air. Titanium oxide nanotube layers were fabricated by an electrochemical anodization of Ti samples in an optimized mixture of DI water + glycerol (volume ratio 50:50) with 0.27 M NH4 F under a constant voltage of Vmax = 10, 20 and 30 V for 2 h (more details in (Pisarek et al., 2013)). After anodization, the samples were rinsed with DI water and dried in air. Subsequently, thermal annealing was performed at 450 ◦ C for 3 h in order to transform the amorphous TiO2 NT structure to crystalline anatase. 2.2. Electron microscopy observations Surface observations of the anodized Ti foils were performed using a Hitachi S-5500 scanning electron microscope. Crosssectional examinations of the layer structure of the nanotubes were carried out with a scanning transmission electron microscope (Hitachi HD 2700) on TEM lamellae. A focused ion beam system (FIB) – Hitachi NB5000 with Ga+ ions beam – was used to mill and extract TEM specimens. The final polishing and cleaning of the sample was performed on a GentleMill low-energy ion polishing system (Technoorg Linda Ltd., Hungary) with an Ar ion beam, operated under an accelerating voltage below 1 kV. 2.3. Tomogram acquisition and reconstruction The electron tomographic studies were performed on nanotubes prepared at anodization voltages of 10, 20 and 30 V. Projection images were acquired using a Hitachi HD-2700 STEM microscope operated at 200 kV. A special 3D tomography Hitachi holder with a 360◦ tilt range was used to acquire the tilt series 2D images. For this holder, a special needle-shaped specimen was required, and so a focused ion beam was used to fabricate the specimen. The procedure of needle-shape sampling includes the first steps of a traditional lift-out preparation, slicing down to around 1 ␮m

and then trimming to a needle-like shape. The final thinning to a diameter of less than 500 nm was performed with a low current (0.07 nA) to avoid damaging the material surface. Tilt-series images were collected in a range of 0–180◦ at increments of 2◦ , using a HAADF detector, using collecting angle of from 70 to 370 mrad that enables images to be obtained with composition contrast. The beam convergence angle during HAADF imaging was 22 mrad. Before each acquisition, the image shift was corrected and the focus adjusted manually. The images were acquired at a magnification of 100,000–130,000 times, with a pixel size of approx. 1 nm, a frame time of 10 s and an image resolution of 512 × 512 pixels. An alignment of the tilt series and the volume reconstruction with a weighted back-projection algorithm was performed with ImageJ software. Visualization of the reconstructed volumes and segmentation were performed with Avizo software. To extract relevant information from the reconstructed volume, a segmentation procedure based on greyscale thresholding was applied. The binary image of the selected material was used to visualize and quantify the structures. An ASAP 2020 sorptometer (Accelerated Surface Area and Porosimetry System, MICROMETRITICS) was used to estimate the specific surface area (Brunauer-Emmett-Teller method – BET) from the krypton adsorption isotherm. The TiO2 nanotubes/Ti samples were carefully degassed in a vacuum at 573 K before Kr adsorption, and cooled to a temperature of 77 K before Kr was admitted to the chamber to measure the isotherm at this temperature. An ASAP 2020 V4.00 computer program was used to estimate the BET specific surface areas. All the corresponding data are normalized with respect to the total weight of the TiO2 nanotube layers, with an estimated value of about 1 mg.

3. Results and discussion Fig. 1a shows an SEM top view image of typical titania nanotubes grown by the anodic oxidation of Ti in an electrolyte at a final voltage of Vmax = 20 V, and subsequently annealed at 450 ◦ C for 3 h. As can be seen, the TiO2 nanotubes (hollow cylinders) are perpendicular to the substrate surface. The average diameter of the tubes, estimated from the image analysis, is 75 nm. The thickness of the nanotube walls is below 20 nm. A cross-sectional view of the TNTs is presented in Fig. 1b. The BF-STEM image reveals three distinct domains: titanium dioxide nanotubes, an interphase region providing adhesion of the nanotubes to the substrate, and the Ti metal substrate. The TNTs fabricated at 10, 20 and 30 V and annealed at 450 ◦ C have a similar arrangement of layers, and vary only in that the size of the nanotubes changes with the voltage in a linear manner, as observed earlier (Macak et al., 2007). The height of nanotubes, determined by the length of the anodic polarization process, was the same for all fabricated samples. As can be seen in the BF-STEM image, the nanotubes are connected by 10–40 nm-thick bridges, and separated by 20–30 nmthick gaps. The side wall ripples which form the bridges between the nanotubes are typical for anodization in a high water content electrolyte (Macak et al., 2008). The reason for the formation of wall ripples is periodic repeated differences in the dissolution and oxide formation rate, which causes some parts of the walls to remain thicker, and others thinner (Macak et al., 2008). The fully crystalline structure of the nanotubes is confirmed by the high-resolution images in Fig. 1b, c, e. This crystalline structure was observed in both the nanotube walls and the bridges connecting two nanotubes. The high-resolution observations revealed also that the nanotubes consist of randomly oriented crystals (Fig. 1c, e) as is shown in the model in Fig. 1d. Such polycrystallinity of TiO2 nanotubes after heat treatment has already been reported in

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Fig. 1. Titania nanotubes structure after annealing at 450 ◦ C (a) top view SEM image, (b) cross-section BF-STEM image, showing nanotube structure in wall, (c) high-resolution STEM image of nanotube wall (d) schematic diagram of randomly oriented TiO2 nanotube, (e) high resolution STEM image of titania nanotube.

other work (Lee et al., 2012), although its formation is not clearly understood. The 3D structural reconstructions of nanotubes fabricated at 10, 20 and 30 V anodization voltage were performed using the electron tomography technique. An example of an FIB prepared needle-shaped sample from titanium anodized at 20 V is presented in Fig. 2. Secondary electron image shows relatively smooth surface of the specimen with some roughness related to existing in the material high porosity (Fig. 2a). As can be seen in bright field STEM and HAADF-STEM images diameter of prepared micropillar is around 500 nm that is enough to visualize in transmission mode small features of TiO2 material (Fig. 2b, c). The interesting part of the specimen is between tungsten mask in the top and titanium substrate at the bottom. Fig. 3 shows HAADF-STEM images of a nanotubes bunch taken at different tilting angles. From the tilt images presented, we can examine whether the observed objects are correctly visualized. One of the most undesirable effects in STEM imaging is the presence of drift during scanning acquisitions of long duration; it results a distortion of the shape of the objects observed. A long acquisition time may also result in the formation of a carbon contamination layer on the specimen that changes the image of the structure. An accurate tilt-series alignment, which is crucial for proper reconstruction, can only be performed on images with no distortions related to drift or contamination. The images presented in Fig. 3 seem to be appropriate for reconstructing the 3D structure of the nanotubes. The variations in visible contrast are related only to the thickness of the material. The reconstructed 3D structure images of the nanotubes fabricated at 20 V are shown in Fig. 4. A 3D voxel projection of the tomographic reconstruction shows the structure of the nanotubes (Fig. 4a). More details on the nanotube morphology can be read

from the orthogonal and longitudinal views (Fig. 4b, c). It can be inferred that the nanotubes are relatively straight and well aligned. The interior surfaces of the walls are rather smooth, in contrast to the external surfaces, which are enhanced by ripples forming bridges between adjoining nanotubes. Measurements performed on the 3D reconstructed images confirm that the size of the bridges ranges from 20 to 30 nm, and the thickness of the adjoining nanotube walls is less than 40 nm. The cross-sectional view of the reconstructed volume provides information about the growth process of the nanotubes. Fig. 4d–f correspond to cross-section planes, marked by arrows in Fig. 4b. The direction in which the nanotubes grow is from the bottom of the image upwards. Thus, the arrow marked “f” indicates the plane at a stage of growth which is earlier than that indicated by arrow “d”. The nanotube structure changes during the growth process, as marked with yellow stars in Fig. 4d–f. At the beginning, there are three separate nanotubes with a diameter ranging from 30 to 70 nm (Fig. 4f). As growth occurs, these three nanotubes coalesce into one (Fig. 4e) structure, and grow with final diameter of 100 nm. The results show that a coarsening effect occurs on the nanotubes during the growth process, and suggest that nanotube diameter becomes a function of the distance from the titanium substrate. The joining of nanotubes during the growth process may result in the formation of a polycrystalline structure of nanotubes during heat treatment. This confirms the previous observation of a polycrystalline structure of nanotubes performed on STEM images. For nanotube cross-sectional observations, the shape is irregular, and far from circular. Such diversity is related to the fabrication process (electrochemical conditions and electrolyte composition) (Roy et al., 2011). Fig. 5 shows images of the reconstructed volume after segmentation. The segmentation process makes statistical estimations of

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Fig. 2. Images of needle-shaped specimen (Ti anodized at 20 V) prepared by FIB for 3D electron tomography reconstruction: (a) SE mode, (b) BF-STEM mode, (c) HAADF-STEM mode.

Fig. 3. HAADF-STEM images of TiO2 nanotubes fabricated at 20 V, showing morphology change for tilt from 0◦ to 160◦ .

Table 1 Morphological parameters, wall thickness (twall ), internal diameter (dtube ), surface area (Stubes ), volume (Vtubes ) determined by 3D image analysis, together with calculated porosity and specific surface area. twall (nm)

10 V 20 V 30 V

5±4 7±3 10 ± 3

dtube (nm) 75 nm

150 nm

700 nm

43 77 145

56 73 149

– – 122

Stubes (nm2 )

Vtubes (nm3 )

porosity (%)

Specific Surface Area(m2 /g)

6.33*105 6.70*105 3.89*106

6.96*106 5.76*106 5.82*107

30 35 55

47.8 30.6 17.5

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Fig. 4. 3D visualization of tomographic reconstruction of TiO2 nanotubes prepared at 20 V, (a) volume rendering of reconstruction, (b) orthogonal slices view through the reconstruction, (c) longitudinal slice view through the reconstruction, (d) cross-section of the reconstructed volume at arrow d, (e) cross-section of the reconstructed volume at arrow e, (f) cross-section of the reconstructed volume at arrow f.

Fig. 5. View of reconstructed volume obtained by segmentation of TiO2 nanotubes obtained at (a) 30 V, (b) 20 V, and (c) 10 V anodization voltage.

morphological parameters such as wall thickness and the internal diameter of the nanotubes possible (Table 1). The results confirm earlier findings obtained by means of SEM image analysis (Macak et al., 2007; Roguska et al., 2016), and show a linear relationship between applied voltage and diameter. As can be seen in Fig. 5a, for an anodization voltage of 30 V, the diameter of the nanotubes ranges from 50 to 200 nm. For some nanotubes the diameter varies with growth, as can be seen in the longitudinal view. The average diameter in relation to distance from the substrate was calculated so as to more precisely describe the variety of nanotube diameter with growth (Fig. 6). The average internal diameter was calculated for 5–10 nanotubes from each anodization state. That calculation was performed on layers perpendicular to the direction of growth of nanotubes in 1–2 nm steps. The internal diameter was determined using the equivalent diam-

eter, defined as the diameter of a circle with the same area as the surface area of the analyzed phase. The result confirmed the variety of nanotube sizes with growth (Fig. 6). The average diameter for an anodization voltage of 30 V decreased from 145 nm close to the substrate to about 120 nm at 700 nm from the substrate (Table 1). In Fig. 5a, it can be seen that some nanotubes contract with growth. The nanotubes obtained at lower voltages of 10 and 20 V were analyzed across a shorter distance, up to 200–300 nm from the substrate, because of finer structure and limited reconstructed volume. As can be seen from the diagram in Fig. 6, the internal diameter increases with the distance from the substrate. However, the average internal diameters measured at 75 and 150 nm from the substrate are higher for 150 nm only in the case of 10 V nanotubes, where that diameter increases from 43 to 56 nm. In the case of nanotubes prepared at 20 V, the internal diameter is similar for 75

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160

Internal diameter [nm]

140 120

30V 20V 10V

100 80 60 40 20 100

200

300

400

500

600

700

800

900

Distance [nm] Fig. 6. Diagram showing the dependence of an average internal diameter on the measured distance from the Ti substrate.

Fig. 7. Specific surface area values obtained from BET measurements and electron tomography reconstruction models.

and 150 nm from the substrate and increases after 200 nm. It should be noted that these observations are based on a limited volume, and so any conclusion with regard to the dependence of nanotube size on distance from the substrate must be treated with caution. The reconstruction of the structure of the sample anodized at 10 V shows that the tubular shape is less obvious than for samples fabricated at higher voltages. In this case, the metal oxide layer has a more porous character, with elongated channels, and so quantification can be more difficult. The thickness of the nanotube walls for all samples was measured in 20 random locations. The results indicate a dependence of the wall thickness on the anodization voltage. The wall thickness for the nanotubes obtained at 10 V was about 5 nm, compared to 7 nm and 10 nm for the nanotubes prepared at 20 V and 30 V, respectively. It should be noted that all of the samples were studied after heat treatment leading to a transformation of the amorphous into a crystalline phase. We suspect that this process may also induce morphology changes, depending on the initial structure, which are different for various anodization voltages. The threedimensional characterization of one titania nanotube in a different state by means of electron tomography has already been successfully applied (Hungría et al., 2009). There, titania nanotubes were prepared by sol-gel and the morphology was analyzed after the transformation of the amorphous phase into the anatase and then the rutile phases. The authors observed differences in the internal and external structure of the object after the transformation to the rutile phase. In our work, the nanotubes are prepared on a titanium substrate, and the rutile phase appears only on the metaloxide interphase as an intermediate layer, as has been previously reported (Hungría et al., 2009). Therefore, only nanotubes of the anatase phase are analyzed here. The segmented sub-volumes were also quantified in terms of surface area and volume, and this made it possible to determine their global porosity and specific surface area. Porosity was measured as a fraction of the volume inside and between the nanotubes to the total volume confined by the nanotubes analysed. The lowest porosity, around 30%, was found for the smallest nanotubes, obtained at 10 V. Porosity increased to 55% for the largest nanotubes obtained at 30 V. This suggests that porosity increases with the diameter of the nanotubes. Along with the increase in porosity, specific surface area decreases, from 47.8 to 17.5 m2 /g, for 10 V and 30 V, respectively. Specific surface area was measured as the total surface area per weight of the volume of nanotubes analysed. This means that the smallest diameter nanotubes obtained at 10 V

have the largest specific surface area. This is so despite the fact that the surface of the pores contributes to the total surface area, and because the specific surface area is primarily controlled by the size of the tubes. It should be noted that the greater specific surface area increases the functional properties of the nanotubes (Hungría et al., 2009). The results of the specific surface area measurements from the 3D model were compared to the estimates based on BET investigations (Fig. 7). The good agreement of the results noted, especially for the larger nanotubes, confirms the applicability of both methods. The BET measurements performed for TiO2 nanotubes fabricated at 20 (20.7 m2 /g) and 30 V (16.2 m2 /g) show a trend in specific surface area which is very close to the results obtained from the electron tomography investigations. In the case of the 10 V sample, the BET method failed to correctly determine the specific surface area, because it was impossible to receive satisfactory physical data (krypton desorption curves). This may be due to the pore diameter being much smaller than in the 20 and 30 V samples, and to the distance between nanotubes. The results presented show that for very fine structures such as 10 V TiO2 nanotubes, electron tomography proved to be a more suitable method for estimating specific surface area. The method described here of measuring porosity and specific surface area is a good supplement to BET measurements, in the sense that it provides both an estimate of and information on subtle elements of the microstructure in question (Hirakata et al., 2010; Crawford et al., 2007). The method might be very useful in the process of designing nanotube-based systems for a variety of applications (Kirchgeorg et al., 2016). The morphology of the TiO2 determines its usefulness as, e.g., a storage material. The large surface area leads to a short Li ions diffusion length into the material, providing a storage capacity of a nanostructured porous material which is much higher than that of most bulk macroscopic materials (Kirchgeorg et al., 2016). Designing a nanostructured material having a tubular geometry which provides gas with ready access to a large specific surface area is one way to improve the catalytic properties of TiO2 materials (Hungría et al., 2009). 4. Conclusions In summary, HAADF-STEM tomography was successfully used to characterize fine structural details such as the internal diameter and wall thickness of TiO2 nanotubes fabricated in an anodization process at different potentials. 3D visualization of the structure of

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the nanotubes also provided an insight into the growth process of the oxide layer. It was found that, with an anodization voltage of 20 V, the nanotubes are initially separated and then coalesce at a later stage of growth. As a result, the diameter of the nanotubes measured close to the substrate can be different than that measured a few hundred nanometers from the substrate. A certain increase in the internal diameter of the nanotubes was observed at a distance of up to 300 nm from the substrate. The finest structure and lowest porosity was observed for the 10 V nanotubes, whose tubular shape is not clearly visible. In these samples the highest specific surface area was measured (47.8 m2 /g). The electron tomography measurements of specific surface area corresponded to the BET measurements of nanotubes obtained at 20 and 30 V. For the smallest nanotubes obtained at 10 V, only the electron tomography technique permitted such fine structure to be characterised. The 3D reconstruction of the structure of nano-structured titania presented here can help us to create a model of morphology having the desired parameters determining the material’s functional properties. Acknowledgment This work was supported by The National Science Centre through the research grant UMO-2014/13/D/ST8/03224. References ´ ´ Andrzejczuk, M., Płocinski, T., Zielinski, W., Kurzydłowski, K.J., 2010. TEM characterization of the artefacts induced by FIB in austenitic stainless steel. J. Microsc. 237, 439–442, http://dx.doi.org/10.1111/j.1365-2818.2009.03288.x. Biermans, E., Molina, L., Batenburg, K.J., Bals, S., Van Tendeloo, G., 2010. Measuring porosity at the nanoscale by quantitative electron tomography. Nano Lett. 10, 5014–5019, http://dx.doi.org/10.1021/nl103172r. Crawford, G.A., Chawla, N., Das, K., Bose, S., Bandyopadhyay, A., 2007. Microstructure and deformation behavior of biocompatible TiO2 nanotubes on titanium substrate. Acta Biomater. 3, 359–367, http://dx.doi.org/10.1016/j. actbio.2006.08.004. Dennenwaldt, T., Wisnet, A., Sedlmaier, S.J., Döblinger, M., Schnick, W., Scheu, C., 2016. Insight in the 3D morphology of silica-based nanotubes using electron microscopy. Micron 90, 6–11, http://dx.doi.org/10.1016/j.micron.2016.08.003. Hernández-Garrido, J.C., Moreno, M.S., Ducati, C., Pérez, L.A., Midgley, P.A., Coronado, E.A., 2014. Exploring the benefits of electron tomography to characterize the precise morphology of core-shell Au@Ag nanoparticles and its implications on their plasmonic properties. Nanoscale 6, 12696–12702, http:// dx.doi.org/10.1039/c4nr03017f. Hindson, J.C., Saghi, Z., Hernandez-Garrido, J.-C., Midgley, P.A., Greenham, N.C., 2011. Morphological study of nanoparticle-polymer solar cells using high-angle annular dark-field electron tomography. Nano Lett. 11, 904–909, http://dx.doi.org/10.1021/nl104436j. Hirakata, H., Ito, K., Yonezu, A., Tsuchiya, H., Fujimoto, S., Minoshima, K., 2010. Strength of self-organized TiO2 nanotube arrays. Acta Mater. 58, 4956–4967, http://dx.doi.org/10.1016/j.actamat.2010.05.029. Holzer, L., Indutnyi, F., Gasser, P., Münch, B., Wegmann, M., 2004. Three-dimensional analysis of porous BaTiO3 ceramics using FIB nanotomography. J. Microsc. 216, 84–95, http://dx.doi.org/10.1111/j.00222720.2004.01397.x. Hungría, A.B., Eder, D., Windle, A.H., Midgley, P.A., 2009. Visualization of the three-dimensional microstructure of TiO2 nanotubes by electron tomography. Catal. Today 143, 225–229, http://dx.doi.org/10.1016/j.cattod.2008.09.014. Kirchgeorg, R., Kallert, M., Liu, N., Hahn, R., Killian, M.S., Schmuki, P., 2016. Key factors for an improved lithium ion storage capacity of anodic TiO2 nanotubes. Electrochim. Acta 198, 56–65, http://dx.doi.org/10.1016/j.electacta.2016.03. 009. Lee, S., Park, I.J., Kim, D.H., Seong, W.M., Kim, D.W., Han, G.S., Kim, J.Y., Jung, H.S., Hong, K.S., 2012. Crystallographically preferred oriented TiO2 nanotube arrays for efficient photovoltaic energy conversion. Energy Environ. Sci. 5, 7989–7995, http://dx.doi.org/10.1039/c2ee21697c.

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