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

Abstract

Ferroelectric materials represent a significant part of functional ceramic materials used in nanoscale electronics. Functional ferroelectric features, such as piezoelectricity and electrostriction, arise from the non-stoichometrical unit cells of ferroelectrics that produce a high spontaneous polarization state. 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). The preparation methods for piezoelectric perovskite structures typically are based on lead and titanium precursors and include sol-gel and hydrothermal methods, organo-metallic chemical vapor deposition (OMCVD), or sputtering. In 1999, self-organized TiO2 nanotubular structures were formed by a simple but optimized electrochemical anodization of titanium in an acidic electrolyte. Later, this anodization approach was significantly improved to achieve the growth of high aspect ratio nanotubes with scalable surface areas in aqueous and organic electrolytes. Meanwhile, the growth of self-organized nanotubular or nanoporous structures has been extended to other valvemetals and even alloys – for an overview see a recent review. These nanotubes combine unique properties of TiO2 with a highly defined nanostructure. This combination opens promising perspectives for applications in solar cells, photocatalysis, catalysis and also in biomedical devices. Recently, using electrochemical deposition under suitable conditions, we have also shown that these nanotube layers can be filled by a secondary 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. 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 bymetal 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. 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 s 1 (see Supporting Information, Fig. S1). Amost 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 (TC1⁄4 490 8C) and ii) a distorted ferroelectric tetragonal lattice below the Curie temperature. 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