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By Jan M. Macak, Cordt Zollfrank, Brian J. Rodriguez, Hiroaki Tsuchiya, Marin Alexe, Peter Greil, and Patrik Schmuki* Ferroelectric materials represent a significant part of functional ceramic materials used in nanoscale electronics.[1] Functional ferroelectric features, such as piezoelectricity and electrostriction, arise from the non-stoichometrical unit cells of ferroelectrics that produce a high spontaneous polarization state.[2] Within this group of materials, lead titanate (PbTiO3) and other perovskite structures (PbZrTiO3, BaTiO3, BiFeO3, SrTiO3) have found use in various devices including non-volatile memory elements, pyroelectric and piezoelectric devices, sensors, actuators that can be integrated into microelectromechanical systems (MEMS).[1–4] The preparation methods for piezoelectric perovskite structures typically are based on lead and titanium precursors and include sol-gel[5] and hydrothermal methods,[6] organo-metallic chemical vapor deposition (OMCVD),[7] or sputtering.[8,9] In 1999, self-organized TiO2 nanotubular structures were formed by a simple but optimized electrochemical anodization of titanium in an acidic electrolyte.[10] Later, this anodization approach was significantly improved to achieve the growth of high aspect ratio nanotubes with scalable surface areas in aqueous[11,12] and organic electrolytes.[13] Meanwhile, the growth of self-organized nanotubular or nanoporous structures has been extended to other valve metals and even alloys – for an overview see a recent review.[14] These nanotubes combine unique properties of TiO2 with a highly defined nanostructure. This combination opens promising perspectives for applications in solar cells,[15] photocatalysis,[16] catalysis[17] and also in biomedical devices.[18] Recently, using electrochemical deposition under suitable conditions,[19,20] we have also shown that these nanotube layers can be filled by a secondary

[*] Dr. J. M. Macak, Prof. P. Schmuki Department of Materials Science and Engineering Chair for Surface Science University of Erlangen-Nuremberg Martensstrasse 7, D-91058 Erlangen (Germany) E-mail: [email protected] Dr. C. Zollfrank, Prof. P. Greil Department of Materials Science and Engineering Chair for Glass and Ceramics University of Erlangen-Nuremberg Martensstrasse 5, D-91058 Erlangen (Germany) Dr. B. J. Rodriguez, Dr. M. Alexe Max Planck Institute of Microstructure Physics Weinberg 2, D-06120, Halle (Salle) (Germany) Dr. H. Tsuchiya Division of Materials and Manufacturing Science Graduate School of Engineering Osaka University, 2-1 Yamada-Oka Suita, Osaka, 565-0871 (Japan)

DOI: 10.1002/adma.200900587

Adv. Mater. 2009, 21, 1–5

material. Several attempts were also made to use hydrothermal treatments of TiO2 nanotubes, to achieve perovskite-type BaTiO3, SrBaTiO3 and PbTiO3 nanotubes.[21] However, in these approaches disorder is often apparent and disbonding during the high-pressure hydrothermal treatment can occur.[21e] In the present work we present an entirely novel approach for synthesis of a highly ordered and vertically aligned piezoelectric lead titanate perovskite nanocellular structure. We first anodize Ti to grow a highly ordered TiO2 nanotube layer. Subsequently, we electrodeposit solid Pb into the nanotubes and finally, we thermally anneal the as-deposited layers in an oxygen flow to obtain a PbTiO3 nanocellular structure that exhibits a piezoelectric behavior. Figure 1 shows a set of scanning electron miscroscopy (SEM) images taken from the TiO2 nanotube layers. In the as-formed state (Fig. 1a), the nanotubes are hollow and have an average diameter and length of 100 nm and 450 nm, respectively. Due to the semiconducting nature of TiO2 and electric leakage in these nanotube layers, it is not possible to fill the TiO2 nanotube layers uniformly by metal electrodeposition, unless the tube bottoms are electrochemically reduced prior to the filling (due to Ti3þ self-doping), thus providing higher conductivity and acting as deposition nucleation sites.[20] But using this pretreatment, Pb electro-deposition inside the tubes can be achieved and most of the nanotubes could successfully be filled (Fig. 1b). A key point is to optimize the filling time at a given filling rate of 8 nm
s1 (see Supporting Information, Fig. S1). A most appropriate time for the deposition under the conditions outlined in the supplementary information was found to be 60 s, as this led to complete filling of nearly all tubes and no significant deposition of Pb on the nanotube layer surface. Subsequently, annealing at different temperatures in an oxygen flow was performed to convert the Pb-filled TiO2 nanotubes into a PbTiO3 perovskite structure. PbTiO3 exists in two crystallographic forms, namely i) a non-ferroelectric cubic lattice above the Curie temperature (TC ¼ 490 8C) and ii) a distorted ferroelectric tetragonal lattice below the Curie temperature.[1d,3,8] The morphology of the samples annealed at different temperatures is shown in Figure 1 for samples annealed at 300 8C (Fig. 1c), 500 8C (Fig. 1d), 550 8C (Fig. 1e) and 600 8C (Fig. 1f). Figure 2 shows XRD spectra taken on the annealed samples. Evidently, the annealing temperature has a significant influence on the structural development and the phase transformation of the Pb-filled nanotubes. After annealing at 300 8C, the structure consists of PbO embedded in the TiO2 phase (with an anatase crystalline structure). Additionally, as seen from Figure 1c, a significantly smaller amount of Pb is present in the TiO2 nanotubes after annealing. This means that a part of the Pb was oxidized to PbO and was partially lost by evaporation. At higher

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Ordered Ferroelectric Lead Titanate Nanocellular Structure by Conversion of Anodic TiO2 Nanotubes

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Figure 1. SEM images of a) TiO2 nanotube layer, b) the same layer after filling with Pb; Pb-filled TiO2 nanotube layers after annealing for 3 hours at a temperature of c) 3008, d) 500 8C, e) 550 8C, f) 600 8C. Images g) and f) show cross-sectional views of the TiO2 and PbTiO3 nanotubes shown in a) and e), respectively.

Figure 2. XRD patterns of the samples shown in Fig. 1c–f annealed at different temperatures.

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temperatures (500 and 550 8C), although a part of the Pb filling evaporated as in the previous case and evidently the tubes become hollow, the annealing process leads to an almost complete conversion to the tetragonal perovskite PbTiO3 structure with a characteristic nanocellular morphology (Fig. 1d and e). The dimensions of the resulting nanocellular structure after annealing at 500 and 550 8C remained almost unchanged in comparison with the former nanotube layer. The thickness of the nanocellular perovskite layer and the average diameter of the cells were approximately 450 nm and 100 nm, respectively. This is particularly evident from the comparison of the cross-sectional images shown in Figure 1g (nanotube layer before the Pb filling) and after conversion to PbTiO3 shown in Figure 1h (nanocellular layer after the annealing). From the XRD spectra (Fig. 2) it is evident that the conversion to a tetragonal perovskite PbTiO3 structure took place at approximately 500 8C. This is in line with literature that reports on temperatures above 500 8C that are required to convert amorphous PbTiO3 (e.g. after sputtering) to a crystalline form,[8] or to convert Pb/TiO2 or PbO/TiO2 systems into crystalline PbTiO3 and avoid formation of other non-perovskite structures.[22] The drawback of an annealing of the Pb-filled TiO2 nanotube structures at too elevated temperatures is evident from Figure 1f. At temperature of 600 8C (Fig. 1f), significant alterations in the tube morphology can be seen due to a sintering and partial collapse of the 3D layer. It should be mentioned that this temperature represents also an upper limit for the stability of the pure TiO2 nanotube layers.[23] Apart from the sintering, a small amount of anatase and rutile TiO2 structure was detected in the layer, probably because at this temperature not only a conversion of the Pb-filled TiO2 nanotubes into PbTiO3 occured, but also the originally amorphous TiO2 was converted to a mixture of anatase and rutile,[23,24] before it could convert to the perovskite PbTiO3, or because Pb evaporated too fast to react with all the TiO2 material. Additional investigations were performed to further confirm that the Pb-filled TiO2 nanotube layers were indeed fully converted into a PbTiO3 perovskite structure. A part of the PbTiO3 nanotube layer shown in Figure 1e,h was stripped-off onto a TEM copper grid by bending the Ti foil in iso-propanol. Afterwards, this piece was investigated by transmission electron microscopy (TEM) together with selected area diffraction (SAED) and spatially resolved energy dispersive X-ray spectroscopy (EDS) in TEM. A representative TEM image of the perovskite nanotube layer is shown in Figure 3a. This example shows a layer that cracked horizontally to the substrate. Clearly four cell units are apparent that represent the tube walls prior to the annealing. This confirms that the original shape of the TiO2 nanotubes remained also in the converted PbTiO3 structure. However, gaps between the individual nanotubes that one would expect are not visible anymore due to an expansion of the material volume upon conversion of TiO2 into PbTiO3 owing to the difference in densities (4.2 g
cm3 for TiO2 vs. 7.8 g
cm3 for PbTiO3). Furthermore, selected area diffraction patterns (SAED) were taken from the walls and confirm the conversion to PbTiO3. An example of a SAED pattern is shown in Fig. 3b; the first five most intense rings correspond to following planes of tetragonal PbTiO3: (001), (101), (111), (002), (200), as also indicated in the figure. The average crystal size of PbTiO3 was estimated to be 16 nm from the peak of (101) plane at 2u 31.68 using the Scherrer equation.[25] This

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Figure 3. a) TEM micrograph of a piece of the PbTiO3 nanocellular layer that has peelled off the Ti substrate, b) SAED ring pattern taken from the walls of the cells shown in a); c) EDS spectra of the samples shown in a).

value is in line with a fact that the average diameter of the PbTiO3 cells does not exceed 25 nm (see Fig. 3a). EDS analysis taken from different parts of the PbTiO3 nanotube layer annealed at 550 8C revealed Pb, Ti and O species in the sample together with a minor fraction of Cu (from the TEM grid) in addition to some carbon content (surface contaminant). After substraction of the last two components, an average atomic % ratio of Pb:Ti:O of 20:20:60 (with an error of approximately 5 at%) was obtained. This is in line with a successful conversion to PbTiO3. However, some variation in the composition (<5%) can be assigned to local differences and a minor presence of PbO that can be seen by XRD. In order to investigate the ferroelectric behavior of the PbTiO3 nanocellular layer synthesized here, we used a modified atomic force microscope (AFM) to measure the local piezoelectric response of the layer as a function of an applied dc bias between the titanium substrate and the conducting AFM tip, as outlined in Figure 4a.[9b,26] Fig. 4b shows an example of the local out-of-plane piezoelectric hysteresis loop measured on the PbTiO3 nanocellular layer obtained by annealing of Pb -filled TiO2 nanotube layers (at 550 8C, shown in Figs. 2e and 3a). Piezoelectric activity and switching in the PbTiO3 layer is shown in Fig. 4b. The limited polarization reversal shown here is due to the intricate field distribution caused by the roughness of the layer surface and the uncertainty of the tip position. In this particular case, the spatial distribution of the applied electric field under the AFM tip is not well-defined and only the effective piezoelectric coefficient can be determined. The effective piezoelectric coefficient, dzz (i.e., field applied and measured along the laboratory z-axis), of the PbTiO3 layer is determined to be approximately 30 pm
V1 from the measured loop. This value is about half of the single crystal value

Adv. Mater. 2009, 21, 1–5

Figure 4. a) Scheme of the set-up used for the piezoelectric measurements; b) typical piezoelectric hysteresis loop of the PbTiO3 nanotube layer.

along the polarization axis (65 pm
V1). The piezoelectric response may be reduced due to substrate clamping effects. For the layers synthesized at temperatures lower than 500 8C, no such loops could be recorded due to the fact that the structure was not ferroelectric perovskite PbTiO3. On the other hand, the samples annealed at 600 8C were almost completely sintered and collapsed (compare situation between Fig. 1e and 1f) which in turn caused difficulties, when placing the tip on the surface and as a result, no reliable loops could be recorded for these samples. In summary, we present a simple and a low-cost approach that can be used to produce highly ordered piezoelectric PbTiO3 nanocellular layers with uniform structure and defined dimension over large areas upon thermal conversion of ordered anodic TiO2 nanotubes layers filled with Pb. The ferroelectric properties of these PbTiO3 nanocellular layers have origin in their highly ordered arrangement that is kept throughout the entire processing. In contrast to the rather tedious approaches, where for example sol-gel approaches[5] and porous alumina membranes[19] (templates) are used for synthesis of perovskites nanostructures,[27] herein we use directly the Pb-filled TiO2 nanotube layers as a part of the desired material – PbTiO3 – without necessity of dissolving the template. Furthermore, within the present work we have established optimal conditions for PbTiO3 nanotube generation – concerning both the tube filling and the annealing conditions. However, it can be expected that changes in piezoelectric properties would occur upon changes in the thickness of the nanocellular layer, the diameter of the cell and the grain size (over a limited range) that could be achieved by

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appropriate design or processing. In this sense, the use of the ordered anodic TiO2 nanotube layers is highly promising, as their dimensions (tube length,[11–14] tube diameter[28]) can be tuned over a very broad range of geometries.

[2] [3] [4] [5]

Experimental Ti foils (0.1 mm thickness, 99.6% purity, Advent Materials, England) were degreased by sonicating in acetone, isopropanol and methanol, followed by rinsing with deionized water (DW) and drying in a nitrogen stream. To form nanotubular layers, the samples were anodized in electrolytes consisting of 1 M H2SO4/0.16 M HF using a conventional three-electrode configuration with a Pt gauze as a counter electrode and a Haber - Luggin capillary with Ag/AgCl (1 M KCl) electrode as a reference electrode. Anodization was done for 2 h at 20 V using a previously published procedure [11]. All electrolytes were prepared from reagent grade chemicals. After anodization, the samples were soaked in deionized water for 24 h to minimize the content of the residual electrolyte. Reductive doping of the nanotubes was carried out in 1 M (NH4)2SO4 at a potential of 1.5 V vs. Ag/AgCl electrode (applied in one step). For Pb deposition we used 1 M Pb(NO3)2 (agitated at 300 rpm) and the current pulsing approach (see Supporting Information Fig. S2). In order to convert the Pb-filled nanotubes to PbTiO3 we thermally annealed the as-deposited samples for 2 h in an oxygen flow in an electrically heated furnace (Heraeus, type K1251) equipped with an alumina tube continously flushed with air at 550 8C, if not denoted otherwise. Scanning electron microscopy (Hitachi FE-SEM S4800) was employed for the morphological characterization of the samples. Transmission electron microscopy (TEM PHILIPS CM 30, operated at 300 kV) was used for the structural characterization of the samples. The TEM was equipped with an EDS detector (Oxford ISIS Link 30, UK). For element analyses a mean electron beam diameter of 8 nm was applied. The phase composition and the crystalline structure of the specimens were determined by X-ray diffractometry (XRD) using monochromatic CuKa radiation at a scan rate of 0.758 min1 over a 2u range of 10–708 (D500, Siemens, Germany). Piezoresponse force spectroscopy was performed using a commercially available atomic force microscope (ThermoMicroscopes Autoprobe CP-Research) with custom tip and sample holders, and PtIr coated tips (Nanosensors, ATEC-EFM) with an elastic constant of about 2.5 N
m1. Local piezoelectric hysteresis loops were measured by positioning the atomic force microscope tip at various sites on the PbTiO3 layer surface and recording the piezoresponse signal as a function of an applied dc voltage superimposed on the probing ac voltage.

[6]

[7]

[8] [9] [10] [11] [12]

[13]

[14] [15]

[16]

Acknowledgements The authors would like to acknowledge Dr. W. Lee for helpful discussions and H. Rollig for valuable technical help. A part of the present work was carried out by using a facility in the Research Center for Ultrahigh Voltage Electron Microscopy, Osaka University, Japan. One of the authors (B.J.R.) acknowledges the support of the Alexander von Humboldt Foundation. Suppporting information is available online from Wiley InterScience or from the author.

[17]

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