Interface Controlled Growth of Single-Crystalline PbTiO3

Aviation and Missile Research, Development, and Engineering Center, US Army RDECOM, Redstone Arsenal, Alabama 35898, United States. J. Phys. Chem...
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Interface Controlled Growth of Single-Crystalline PbTiO3 Nanostructured Arrays Hyun-Cheol Song,†,‡ Deepam Maurya,‡,∥ Mohan Sanghadasa,⊥ William T. Reynolds, Jr.,§ and Shashank Priya*,‡,∥ †

Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea Center for Energy Harvesting Materials and System (CEHMS), §Department of Materials Science and Engineering, and ∥Institute for Critical Technology and Applied Science (ICTAS), Virginia Tech, Blacksburg, Virginia 24061, United States ⊥ Aviation and Missile Research, Development, and Engineering Center, US Army RDECOM, Redstone Arsenal, Alabama 35898, United States ‡

S Supporting Information *

ABSTRACT: PbTiO3 (PTO) ferroelectric perovskite has appealing electromechanical characteristics such as low aging rate of the dielectric constant, a high pyroelectric coefficient, a high piezoelectric voltage constant (gij), and a high Curie temperature of 490 °C. However, the high tetragonality of PTO ceramics makes them difficult to be synthesized via conventional high-temperature techniques. Here, a novel synthesis methodology is reported that results in epitaxial growth of a vertically aligned array of PTO nanofibers on a Ti metal substrate. High quality single crystal PTO nanofibers oriented along the [001] PTO direction were obtained on a (110) oriented TiO2 seed layer using a low-temperature hydrothermal synthesis technique. Fundamental understanding of the nucleation and growth criterion is provided through a combination of modeling of the geometric matching of crystal surfaces and experiments detailing the role of underlying TiO2 phase and interplanar atomic configuration. Crystal matching revealed good correspondence at an interface between parallel PTO (001) and rutile TiO2 (110) planes, in which six rows of Ti atoms in (010)PTO-type planes match with approximately seven rows of Ti atoms in (11̅0)rutile-type planes. In the orthogonal direction within the interface, four rows of Ti atoms in (1̅00)PTO-type planes correspond to five Ti atoms in (001̅)rutile-type planes. The lattice-matched interface appears to facilitate nucleation of epitaxial nanofiber growth. Availability of single crystalline PTO dense nanofiber arrays can give rise to a new generation of sensing and high-temperature energy harvesting applications. piezoelectric constant (dij) of ZnO (d33 = ∼26.7 pm/V) is very low compared to typical piezoelectric materials with perovskite structure.28 Thus, there is desire to fabricate piezoelectric nanostructures with the perovskite crystal structure and high piezoelectric properties from compositions such as PbTiO3, Pb(Zr,Ti)O3, Pb(Mg,Nb)O3−PbTiO3, BaTiO3, KNbO3, etc. Among these different perovskite piezoelectric materials, PbTiO3 (PTO) has some appealing characteristics such as a low aging rate of the dielectric constant, a high piezoelectric voltage constant (gij), and a high Curie temperature of 490 °C.29,30 Also, PTO has high tetragonality (c:a ratio of 1.063), which can provide a low ratio for the planar-to thickness coupling factor.31 However, the high tetragonality also makes PTO ceramics difficult to be synthesized via conventional hightemperature synthesis. As PTO undergoes ferroelectric phase transition during cooling through the Curie temperature, it experiences a large expansion in the crystallographic c-direction

1. INTRODUCTION Over the past few decades, one-dimensional (1-D) piezoelectric nanomaterials including nanorods,1−3 nanotubes,4 nanobelts,5 and nanofibers6 have been studied extensively because of their unique physical and chemical properties such as a large surface area,7 excellent charge transport,8,9 high electromechanical coupling,10 and superior ferroelectric properties.11 One-dimensional nanostructures have a high aspect ratio and an anisotropic geometry that can provide higher sensitivity as compared to planar configurations.12 Furthermore, one-dimensional piezoelectric nanostructures are promising candidates for nonvolatile memories,13,14 pressure sensors,12,15−18 actuators,19 biointerfaced mechanical probes,20 and nanogenerators.21−27 In this paper, we report one-dimensional epitaxial piezoelectric nanofiber arrays grown on a rutile template and discuss their growth mechanism. Among the piezoelectric nanomaterials, semiconducting ZnO nanowires are the most extensively studied material over the past decade. Vertically aligned single crystal ZnO nanowires can be grown with a preferred direction and without any seed layer owing to the hexagonal wurtzite structure. However, the © 2017 American Chemical Society

Received: September 21, 2017 Revised: November 1, 2017 Published: November 7, 2017 27191

DOI: 10.1021/acs.jpcc.7b09369 J. Phys. Chem. C 2017, 121, 27191−27198

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The Journal of Physical Chemistry C

Figure 1. (a) Schematic illustration of fabrication process of PbTiO3 nanofibers on Ti substrate with rutile TiO2 seed layer: (i) electrolytic polishing of Ti foil, (ii) TiO2 seed layer deposition by the sol−gel method, and (iii) PbTiO3 nanofiber growth by hydrothermal synthesis. (b) Surface SEM image of electrolytic polished Ti foil. (c) Surface SEM image of TiO2 seed layer on Ti foil annealed at 700 °C for 5 min. (d) Vertically grown PbTiO3 nanofiber arrays on TiO2(rutile)/Ti substrate by hydrothermal synthesis.

mirror finish and ultrasonically cleaned in Micro-90 clean solution, sequentially rinsed with acetone, isopropanol, and methanol prior to electrolytic polishing. Titanium anode and platinum cathode sheets, 1 cm × 1 cm in size, were electropolished at ∼1 °C in an agitated solution of glacial acetic acid and perchloric acid (9:1 volume ratio) at 55 V for roughly 2 min.44 The electrolytic polishing was conducted at low temperature (below 1 °C) in consideration of the polishing plateau that broadens with decreasing temperature.45 The polished Ti surface was examined with SEM to assess the topography of the electropolished foil surface. 2.2. TiO2 Seed Layer Growth. A TiO2 sol−gel solution for the seed layer was prepared using a previously reported process.44 About 369 μL of titanium isopropoxide (99.99% pure, Sigma-Aldrich) was mixed with 2.53 mL of ethanol. In a separate vial, 35 μL of 2 M HCl solution (99% pure, SigmaAldrich) was added to 2.53 mL of ethanol. This HCl solution was then added dropwise to the titanium isopropoxide solution under 500 rpm stirring for 1 h, and then the mixture was filtered using a PTFE 0.2 μm filter. The prepared TiO2 sol−gel solution was spin-coated on the electropolished Ti foils at 6000 rpm for 60 s. After drying at 120 °C for 10 min, the TiO2 films were annealed at 450, 650, and 800 °C for 10 min. Annealing was conducted in air under atmospheric pressure in a tube furnace. The morphology and structure of the annealed films were examined with FESEM (LEO 1550) and XRD (Bruker D8 advance). 2.3. PbTiO3 Nanofiber Growth by Hydrothermal Synthesis. Chemical grade tetrabutyl titanate ((C4H9O)4Ti) and lead nitrate (Pb(NO3)2) were used as reactants, and potassium hydroxide (KOH) was employed as a mineralizer. Poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) were added as a polymer surfactant to facilitate one-dimensional growth. First, (C4H9O)4Ti was distilled with high purity ethanol under 200 rpm stirring for 10 min. Next, this solution was precipitated in a 0.1 M ammonia solution. The precipitate was filtered and washed with distilled DI water for 10 min in order to remove ammonium ions and ethanol. The TiO(OH)2 precipitate was redispersed with 50 mL of DI water to a 0.1 M

and contraction in the two orthogonal crystallographic directions. These strains generate internal stress and cause microcracks.32 It has been suggested that the cracking problem can be avoided by fabricating ferroelectric PTO ceramics below its Curie temperature (490 °C) through low-temperature synthesis routes.33 A number of studies on PbTiO3 nanostructures have been reported; however, most of these investigations are related to randomly oriented polycrystalline material34−36 or powder if synthesized as a single crystal.37−41 Only a limited number of studies have been conducted on vertically aligned crystals grown on semiconducting or metallic substrates.42,43 A vertically aligned piezoelectric nanostructure array offers many advantages such as enhanced mechanical to electrical energy conversion efficiency and simpler fabrication of devices such as nanogenerators and sensors. Moreover, the mechanisms associated with the growth of single crystal PTO nanostructures on TiO2 seed crystals are not yet clear. Here, we report the epitaxial growth of vertically aligned arrays of PTO nanofibers on a Ti metal substrate coated with a rutile TiO2 seed layer. High quality single crystal PbTiO3 nanofibers oriented along the [001] PTO direction were obtained on an (110) oriented TiO2 seed layer using a lowtemperature hydrothermal synthesis technique. The seed layer was synthesized by electrolytically polishing Ti foil, spin-coating sol−gel TiO2 seed crystals, and annealing under controlled conditions. The vertically aligned PTO nanofiber arrays were densely packed and grown over a large area of the TiO2/Ti foil substrate. The crystalline phase, orientation, and morphology of the PTO nanofibers were confirmed with high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Furthermore, a systematic study of the growth mechanism of the PTO nanofibers on the rutile TiO2 seed layer is provided by modeling the interfacial matching between PTO and TiO2 atomic surfaces.

2. EXPERIMENTAL SECTION 2.1. Electrolytic Polishing of Ti Substrate. Titanium foils (99.99% pure, Sigma-Aldrich) were mechanically polished to a 27192

DOI: 10.1021/acs.jpcc.7b09369 J. Phys. Chem. C 2017, 121, 27191−27198

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Figure 2. (a) 3-D atomic structures of interface of TiO2 (110) rutile and PbTiO3 (001) surfaces. (b) Schematic illustration of in-plane atomic structure between (110)rutile and (001)PTO surfaces which are in parallel. Along the horizontal direction, six Ti atoms in (01̅0)PTO-type planes match seven Ti atoms, (1̅10)rutile-type planes. Similarly, along the vertical direction, four Ti atoms in (01̅0)PTO-type planes correspond to five Ti atoms in (1̅10)rutile-type planes. Blue or light blue ball: Ti; red or pink ball: O; yellow ball: Pb.

white arrows in Figure 1c. Using this TiO2/Ti substrate, PTO nanofibers were synthesized hydrothermally at 225 °C for 12 h. The vertically aligned PTO nanofiber array obtained from the TiO2 seeds is shown in Figure 1d. The mean diameter and length of PTO nanofibers were measured from a magnified SEM image and found to be approximately 200 nm and 10 μm, respectively. The aspect ratio of the PTO nanofibers was mostly larger than 50 and can be controlled by adjusting PVA/PAA surfactant ratio and quantity.46 Because of the high aspect ratio, the fibers tend to bend slightly outward around seed spots. Furthermore, the vertically aligned PTO nanofiber arrays can be fabricated over a large area through formation of a homogeneous TiO2 seed layer on Ti foil (see Figure S1a,b). 3.2. Growth Mechanism of Epitaxial PbTiO3 Nanofibers on Rutile Seeds. From our experimental results, it is clear that the TiO2 seed layer plays an important role in the growth of single crystal PTO nanofibers during solution-based synthesis. In general, an epitaxial substrate, or template, should have a very similar crystal structure to that of the phase being grown. In the case of TiO2, however, the atomic structure is quite different from that of PTO perovskite, and it raises the question as to why the TiO2 template layer is effective for seeding growth of PTO nanofibers. In order to explore the answer of this question, we searched for good-matching interfaces between TiO2 and PTO phases using geometric matching. This strategy superimposes the atom positions of the two phases in three dimensions and employs a coincidence criterion to identify good-matching interfaces.47 This approach accurately reflects the periodicity in a given interphase boundary, but not the actual atomic positions of a relaxed boundary. Nevertheless, geometric matching is useful for identifying crystal planes that have compatible charges and thus can form a plausible interface between the two phases. The rutile structure was chosen for the TiO2 phase in matching model because it is most similar to PTO perovskite structure among several possible TiO2 polymorphs. An automated algorithm was implemented to determine the quality of geometric matching between rutile and PTO for 106 random orientation relationships. The best matching orientation

concentration, and a stoichiometric amount of Pb(NO3)2 was added to this aqueous solution with vigorous stirring. Pellets of KOH were slowly introduced to a 2 M concentration. Finally, the polymer components, 0.02 g of PVA and 0.4 g of PAA, were added to the solution and stirred for more than 12 h. Hydrothermal synthesis was performed in a stainless steel autoclave with a 23 mL volume Teflon vessel (Parr Instrument Company). Titanium foil with the TiO2 seed layer was vertically suspended in the solution from a Teflon holder. The Teflon vessel was filled with 20 mL of hydrothermal solution, sealed in a stainless steel autoclave, and placed in a low-temperature furnace at 225 °C for 12 h. After cooling to room temperature, the synthesized specimens were washed several times with DI water and high purity ethanol (99.9%) and dried in the air. The fabricated piezoelectric nanostructures were characterized by XRD (Bruker D8 advance), FESEM, EDX (LEO 1550), Raman spectroscopy (JY Horiba LabRam HR800-U), and HRTEM (FEI Titan 300)

3. RESULTS AND DISCUSSION 3.1. Fabrication of Epitaxial PbTiO3 Nanofiber Arrays. A schematic of the synthesis sequence for fabricating the array of PTO fibers is depicted in Figure 1a. An SEM image of the electropolished Ti foil, shown in Figure 1b, demonstrates that the polished surface is flat. Figure 1c shows the surface of a TiO2 rutile seed layer after spin-coating on the electropolished Ti foil and annealing at 700 °C for a short time (5 min, to avoid excessive oxidation of the Ti substrate). The as-deposited surface of the TiO2 in Figure 1c, shown at a much lower magnification than the polished Ti surface of Figure 1b, is clean and flat with TiO2 grains on the order of a few tens of nanometers. The large grainlike regions indicated by the white dashed lines in Figure 1c appear to be related to the grain structure of the underlying Ti foil. The size of the TiO2 grains differ from one underlying Ti grain to the next. The different grain sizes of TiO2 could be attributed to a growth rate dependent on the orientation of the Ti foil surface. The actual grain size of TiO2 seeds varied between approximately 20 and 50 nm over different underlying Ti grains as indicated by the 27193

DOI: 10.1021/acs.jpcc.7b09369 J. Phys. Chem. C 2017, 121, 27191−27198

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Figure 3. (a) XRD patterns of TiO2/Ti substrate annealed at 450, 650, and 800 °C. (b) TiO2 phase diagram with temperature and pressure. Reproduced with permission from ref 58. Copyright 2000 Springer.

relationship was found to be within one degree of the set of parallel directions: [01̅0]rutile//[001]PTO and [1̅01]rutile// [110]PTO. The normal to the (110)rutile plane is a preferred growth direction under typical conditions because the surface entropy of the (110)rutile plane is known to be much smaller than that of the (001)rutile or (101)rutile surfaces ((101̅) and (101) have same surface entropy). The calculated surface energy values of (110), (100), and (101) are 2.61, 3.69, and 4.33 J/m2, respectively.48 Given that a (110)rutile plane is the most stable surface, we selected that plane as the substrate to match with PTO. The most plausible matching configuration for O-terminated rutile TiO2 (110) and Ti-terminated PTO (001) surfaces is shown in the perspective view of Figure 2a. The plane view of the interface between a single rutile TiO2 (110) plane and a Ti-terminated PTO (001) plane is depicted in Figure 2b. The lighter blue and red balls represent titanium and oxygen atoms in the rutile TiO2 crystal, and the darker blue and red balls represent titanium and oxygen atoms in the PTO crystal, respectively. Along the horizontal direction of Figure 2b, six rows of Ti atoms in (010)PTO-type planes match with approximately seven rows of Ti atoms in (11̅0)rutile-type planes. Similarly, along the vertical direction in Figure 2b, four rows of Ti atoms in (10̅ 0)PTO-type planes correspond to five Ti atoms in (001̅)rutile-type planes. While the lateral growth of a PTO thin film on rutile TiO2 should be constrained by lattice misfit, a thin nanofiber can presumably accommodate some of this misfit through coherency strain.49 Thus, we can expect that one-dimensional PTO nanofibers can grow dislocation-free single crystals more readily than two-dimensional PTO thin films. The lattice-matched O-terminated Ti sites in rutile TiO2 are likely to play an important role in the nucleation of PTO that leads to epitaxial growth of the [001] orientation of PTO nanofibers. At elevated temperature and pressure in hydrothermal conditions, the nuclei of crystalline PTO can be formed at the lattice matched site on the rutile TiO2 by precipitation of the solute ions according to the following reaction:

The PTO nuclei grow and gradually evolve to a facet nanocrystal with cube shape. The dissolved ions reprecipitate on surfaces of the nanocrystal grown in all directions except at conjoined facet. As the nanocrystal increases in diameter and thickness, the growth of PTO crystal is temporarily hindered by adsorption of surfactants (PVA/PAA) on the surface through hydrogen bonding. The surfactant encapsulation of an exposed (001) facet, which has the lowest surface energy, can be broken to form chemical bonds with growth components, while the other facets are hindered by steric surfactant arising from surface energy differences. Because of a presumably large anisotropy in growth rate with respect to surface orientation, the nanocrystals formed at nucleation sites elongate preferentially and lead to nanofiber growth. Additionally, the presence of a dipole in the growth components may facilitate onedimensional growth in the [001] direction by dipole-induced attractions.50 The nanofiber growth is accomplished by reduction of the concentration of the growth components until equilibrium is reached. 3.3. Experimental Validation of PbTiO3 Nanofiber Growth Mechanism. To validate the geometric modeling results for lattice matching of rutile (110) TiO2 and (001) PTO, we fabricated PTO nanofibers with varying TiO2 phases by changing annealing temperature. Figure 3a shows the X-ray diffraction (XRD) patterns of TiO2 seed layers on Ti foil after annealing at different temperatures. The TiO2 films were fabricated via the sol−gel chemical solution technique and annealed at 450, 650, and 800 °C for 10 min. To prevent excess oxidation of the Ti foil substrate during TiO2 seed layer formation, the specimens were also quickly cooled to room temperature in air by taking them out of the furnace as soon as the annealing process was completed. The rutile TiO2 phase was formed above 650 °C annealing temperature as shown in the XRD pattern in Figure 3a. With increasing annealing temperature from 650 to 800 °C, the intensity of the XRD peak increased considerably, indicating a greater degree of crystallinity of the rutile TiO2 film. As shown in Figure 3a, the (110) orientation peak is the most dominant which indicates that the TiO2 film on the underlying Ti foil annealed at 800 °C has a strong (110) preferred orientation and 96% texturing is found to be along this orientation (calculated by

HPbO2−(aqueous) + Ti(OH)y 4 − y (aqueous) → PbTiO3 + 2H 2O 27194

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Figure 4. Surface SEM images of TiO2 seed layers on Ti plate annealed at (a) 450, (b) 650, and (c) 800 °C for 10 min. SEM images of PbTiO3 nanofibers grown on TiO2/Ti substrates annealed at (d) 450, (e) 650, and (f) 800 °C for 10 min by hydrothermal synthesis at 225 °C for 12 h. (g), (h), and (i) are magnified images for PbTiO3 nanofibers in (d), (e), and (f), respectively.

Figure 5. TEM images and XRD patterns of PbTiO3 nanofibers hydrothermally grown on TiO2/Ti substrate at 225 °C for 12 h. (a, b) TEM images of individual PbTiO3 nanofiber. (c) HRTEM image and SAED pattern (inset) of outside edge of PbTiO3 fiber. (d) XRD patterns of PbTiO3 nanofibers grown on Ti substrate with rutile TiO2 seeding. (e) Raman spectra of PbTiO3 nanofibers at room temperature. Inset is Raman spectra of PbTiO3 nanofibers in the vicinity of Curie temperature (400, 450, and 500 °C).

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peaks, indicating that the samples are well crystallized. All of the diffraction peaks can be indexed as the tetragonal perovskite structure of PTO which was in exact agreement with JCPDS Card No. 77-2002. This confirms that the fabricated PTO nanofiber arrays have a single perovskite structure without any trace of a second phase. In addition, the EDS analysis shown in Figure S3 verifies that the nanofibers are the stoichiometric PbTiO3 phase. The high-resolution TEM image in Figure 5c shows clear lattice fringes, demonstrating that the PTO nanofibers are structurally uniform and well crystallized. The observed lattice−fringe distances of 4.10 and 3.87 Å are in agreement with the d-spacing of the tetragonal (001) and (101) planes, respectively. The selected area electron diffraction (SAED) pattern (Figure 5c, inset) shows clear diffraction spots of PTO crystals obtained from nanofiber region in Figure 5c, which unambiguously confirms the single crystalline nature of the nanofiber. To understand the lattice vibrations in PTO nanofibers, a Raman scattering investigation was performed at various temperatures. The Raman scattering spectrum of PTO nanofibers recorded at room temperature (20 °C) is shown in Figure 5e. All Raman modes of the PTO nanofibers were indexed based on the earlier study of Burns and Scott.54 The wavelength of Raman modes in the nanofiber agrees well with the previously reported spectra of the tetragonal PTO phase.55,56 At room temperature, PTO has a tetragonal unit cell with P4mm point group symmetry, and the zone-center optical phonons are grouped as five A1, six E, and one B1 symmetry, all of which are Raman-active. One of the soft phonon modes, E(1LO), was observed at 124.4 cm−1, which indicates that the nanofibers are polar at room temperature. The variation in the PTO nanofiber Raman spectra with temperature from 400 to 500 °C is shown in the inset of Figure 5e. As temperature increases, the Raman lines downshift and significantly broaden compared with the room temperature spectrum. At 450 °C, the intensity of the Raman lines drops dramatically and almost vanishes at 500 °C. When we consider that near the Curie point short- and long-range interactions compensate each other in such a way that the frequency of a transverse optical (TO) phonon approaches zero, the result indicates that the tetragonal-to-cubic phase transition (ferroelectric-to-paraelectric) occurs between 450 and 500 °C. This result is in agreement with the previous Raman study and typical Curie temperature of PTO.57 It is concluded that this ferroelectric transition is clear evidence of ferroelectric behavior of the PTO nanofibers.

peak intensity comparison). This agrees with previously reported results that the (110) facet is the most stable for the rutile TiO2.51,52 The TiO2 film annealed at 450 °C for 10 min did not show any other perceptible XRD peaks except those from the Ti substrate. This might be because the annealing time is too short to form the anatase phase at low temperature. The XRD results correspond well with the known TiO2 temperature−pressure phase diagram (Figure 3b). Surface SEM images in Figures 4a, 4b, and 4c show the TiO2 seed layer deposited on the electropolished Ti foils annealed at 450, 650, and 800 °C for 10 min, respectively. The TiO2 film annealed at 450 °C shows no visible grains, which is consistent with the XRD data of Figure 3a. The average grain size of the TiO2 films was determined from magnified SEM images (Figures S2a−c) and found to be 50−200 nm at 650 °C and 200−500 nm at 800 °C. Different mean grain sizes were observed over different grains of the underlying Ti substrate as indicated by the white arrows in Figure 4b. With increasing annealing temperature up to 800 °C, the grain size of TiO2 was significantly increased, and abnormal grain growth was also observed (indicated by white circles in Figure 4c). Figures 4d−f show SEM images of PTO nanofibers grown on Figures 4a−c TiO2 templates by hydrothermal reaction at 225 °C for 12 h. As expected, the TiO2 phase formed at low temperature did not achieve one-dimensional PTO nanostructure. On the other hand, the hydrothermal growth using the rutile TiO2 seed layers formed above 650 °C resulted in the bushlike PTO nanostructures as shown in Figures 4e,f. A relatively rough surface and abnormal grain growth of the TiO2 layer without electrolytic polishing resulted in the formation of the bushlike nanostructures instead of vertically aligned crystals. Figures 4h and 4i show magnified SEM images of the bushlike PTO nanostructures with the rutile TiO2 seed layers annealed at 650 and 800 °C, respectively. The length and density of the bushes further increased with increasing annealing temperature of the TiO2 seed layer. Each nanofiber has rectangular cross-sectional geometry. The nanofibers grown on the 650 °C TiO2 seed layer showed a larger mean diameter of ∼400 nm than that of the 800 °C TiO2 seed layer, having a diameter of ∼250 nm. The closely packed nucleation sites could obstruct the planar growth of nanofibers and lead to smaller diameters. Thus, dense PTO nanobushes would have smaller diameter nanofibers. From the XRD patterns for TiO2 seed layers and SEM images of PTO nanofibers, we can clearly confirm that only rutile TiO2 layers serve as a suitable seed to guide the growth of the PTO nanofibers. 3.4. Characteristics of PbTiO3 Nanofibers. Furthermore, morphological and structural characterizations of the PTO nanofiber arrays were performed using transmission electron microscopy (TEM). Figures 5a,b show typical TEM images of a PTO nanofiber 200 nm in diameter. As shown in Figures 5a−c, individual PTO nanofibers have continuous lattice fringes along the growth direction and no obvious grain boundaries, defects, or dislocations. Interestingly, a dark line is frequently observed along the middle of nanofibers along the longitudinal direction. This contrast line is a ferroelectric 180° domain similar to that observed in PZT nanofibers.53 This type of domain is strong evidence that the PTO nanofibers are ferroelectric. The TEM results also confirm that the PTO nanofibers are single crystals along their entire length, and they grow along the [001] direction. This result closely agrees with XRD pattern of the PTO nanofiber arrays on TiO2/Ti foil as shown in Figure 5d. The XRD patterns in Figure 5d exhibit very sharp diffraction

4. CONCLUSIONS In summary, vertically aligned single crystal PTO nanofiber arrays have been grown on a metal Ti substrate via rutile TiO2 seeding. From geometric modeling of crystal surfaces, rutile TiO2 (110) and PTO (001) planes provide a good-matching interface. Six (010)PTO-type planes match with approximately seven (110̅ )rutile-type planes across the interface. Along an orthogonal direction, four (1̅00)PTO-type planes match five (001̅)rutile-type planes across the interface. The lattice-matched interface plane could provide an effective nucleation site for epitaxial nanofiber growth. Furthermore, the experimental results confirm that only the rutile polymorph of TiO2 serves as a template for PTO nanofiber epitaxial growth and the stable (110) surface of rutile TiO2 provides (001) growth of PTO nanofibers. 27196

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09369. Further details of SEM images of TiO2 seed layer and PbTiO3 nanofibers, EDS analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(S.P.) E-mail: [email protected]. ORCID

Hyun-Cheol Song: 0000-0001-5563-9088 Author Contributions

H.-C.S. conceived the idea and fabricated the nanofibers. H.C.S. prepared the main manuscript. H.-C.S. and D.M. characterized properties of the nanofibers through TEM and SIMS. W.T.R. carried out 3-dimensional geometric matching using computer modeling. M.S. and S.P. initiated and supervised research providing suggestions throughout the study. All authors discussed the results and have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from Office of Basic Energy Science, Department of Energy (D.M. and S.P., grant # DE-FG02-06ER46290), NSF CREST program (H.-C.S., grant # HRD 1547771), the Korea Institute of Science and Technology (H.-C.S., grant # 2E27120), and NSF (W.T.R., grant # DMR-1506936).

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REFERENCES

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