Surface Symmetry Effect on Self-Assembly of Three-Dimensional

Mar 27, 2018 - ... of Science and Technology (KIST), Seoul 02792 , Republic of Korea ... and Engineering Center, U.S. Army RDECOM, Redstone Arsenal, ...
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Communication Cite This: Chem. Mater. 2018, 30, 2183−2187

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Surface Symmetry Effect on Self-Assembly of Three-Dimensional Single Crystal Piezoelectric Nanostructures Hyun-Cheol Song,† Min-Gyu Kang,‡ Deepam Maurya,‡ Mohan Sanghadasa,§ Robert J. Bodnar,∥ 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), Bio-Inspired Materials and Devices Laboratory (BMDL), Virginia Tech., Blacksburg, Virginia 24061, United States § Aviation and Missile Research Development, and Engineering Center, U.S. Army RDECOM, Redstone Arsenal, Huntsville, Alabama 35898, United States ∥ Department of Geosciences, Virginia Tech., Blacksburg, Virginia 24061, United States ⊥ Department of Materials Science and Engineering, Virginia Tech., Blacksburg, Virginia 24061, United States ‡

S Supporting Information *

T

paper, we report a novel interface controlled growth to achieve functional 3-D nanostructures and provide the fundamental understanding of the growth process. The (001) oriented PTO single crystal arrays, with well-defined geometrical shapes, were grown on TiO terminated (001) SrTiO3 (STO) substrates through a hydrothermal synthesis. The growth mechanism of the 3-D PTO nanostructures was found to be governed by the atomic interface between the PTO and underlying TiOterminated STO substrate. The crystallinity, orientation, and morphology of 3-D PTO nanostructures were investigated using a high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and energy-dispersive X-ray spectroscopy (EDX). The piezoelectric behavior of the 3-D PTO nanostructure was demonstrated using piezo response force microscopy (PFM). The STO has the same crystal structure as PTO and a comparable a- and b-axis lattice parameters, which makes it a suitable substrate with least lattice mismatch. The a- and b-axis lattice constants of PTO (3.904 Å) have almost no mismatch with the corresponding axes of STO (3.905 Å), which favors epitaxial PTO growth in the [001] direction on (001) STO substrate. The growth procedure is schematically illustrated in Figure 1a−c. On the Ti−O terminated surface, the nuclei are formed following the crystallographic atomic arrangement (Figure 1a). The nanocrystals grow from the nuclei in an ordered linear arrangement following the directionality imparted by the Ti−O surface (Figure 1b). The growth is preferentially 2D due to large anisotropy in growth rate with respect to surface orientation, which mainly results from the steric surfactants of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) (Figure 1c). Figure 1d shows an atomic force microscopy (AFM) image of the STO surface. The atomic terrace structure was observed, which confirms that the STO surface has TiO-termination.26 Representative SEM images of 3-D nanostructured PTO crystals grown using a hydrothermal synthesis process on the TiO-terminated (001) STO substrate

he past few decades have seen rising interest in the threedimensional (3-D) nanostructures because of their unique characteristics, such as large surface area,1 novel functional behavior2 and better physical/chemical properties resulting from nanoscale effects.3−6 The 3-D nanostructures are attracting interest for applications in sensors, nanotemplates, electronic materials, transistors, catalysts and battery electrodes. There are two main methodologies for fabricating 3-D nanostructures, namely, soft-template assisted growth and self-assembly. The soft-template assisted fabrication process for 3-D nanostructures has advantages of scalability and precise microstructure control.7−9 However, it is difficult to fabricate high quality 3-D nanostructures, especially single crystal structures, using soft-template assisted methods because of limitations in developing recipes, and in separating the fabricated structures from the template.10−13 Self-assembly provides a practical pathway toward overcoming these limitations, and realizing 3-D single crystalline nanostructures, while enabling the control of growth orientation.1,13−17 Here, we provide a breakthrough in this direction by exploiting interface driven growth that assists in relaxing the interfacial strain through shape modulation, resulting in piezoelectric 3-D nanostructures. Piezoelectric 3-D nanostructures provide additional advantages that arise from their anisotropic functional properties.18−21 Among various known piezoelectric materials, PbTiO3 (PTO) has some appealing characteristics such as a low aging rate of the dielectric constant, a high voltage constant (g), and a high Curie temperature of 490 °C.22,23 Also, PTO has high tetragonality (c:a ratio of 1.063) that can provide a low ratio for the planar-to-thickness coupling factor.24 Photocatalytic characteristics of PTO for the production of O2 and H2 by splitting water have been explored recently.25 However, synthesis of PTO 3-D nanostructures has been challenging and few successes have been reported. PTO exhibits high intrinsic strain during cooling from high temperature to room temperature, which normally fractures the ceramic material. Overcoming this issue, in the synthesis of PTO nanostructures, without compromising the electromechanical properties, is highly desired but has been fundamentally difficult. In this © 2018 American Chemical Society

Received: January 11, 2018 Revised: March 9, 2018 Published: March 27, 2018 2183

DOI: 10.1021/acs.chemmater.8b00135 Chem. Mater. 2018, 30, 2183−2187

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Figure 1. Schematic illustration of the growth mechanism of 3-D PTO nanostructures (a−c) by hydrothermal process (a) TiO-terminated (001) STO substrate. (b) Nucleation dominated along the TiO atomic sites in [100] and [010] direction of (001) STO substrate. (c) Crosshatch patterned 2-D nanosheet growth along c-axis of STO. (d) Topological AFM image of atomic terrace structure of TiO-terminated STO. (e, f) SEM images of 3-D STO nanostructures with aligned cross-links.

are shown in Figure 1e,f. Figure 1f is the magnified image of 3D PTO nanostructures. The PTO nanocrystals are arranged in a 3-D array with a cross-hatched pattern that suggests 4-fold rotational symmetry. The individual PTO grains correspond to two-dimensional (2-D) PTO nanosheets that are periodically aligned in a cross-hatched pattern. The 2-D nanosheets have a rectangular plate-morphology with 1 μm length and width and 50 nm thickness, and the plates are aligned along the [100] and [010] directions of the (001) STO substrate. As shown in Figure 1f, the average spacing of the PTO plates is approximately 1 μm, and is roughly the same in both alignment directions. In some cases, the large intersecting plates enclose a group of smaller plates, which are caused by the geometrical constraint imposed by large plates on the subsequent growth of smaller plates. A fraction of the 2-D nanosheets grew at a 45° angle to the dominant alignment direction and intersected each other at right angles as shown in the magnified image in Figure 1f. Figure S1a (Supporting Information) demonstrates the 3-D PTO nanostructure has a uniform distribution over a relatively large area of the STO substrate. In order to investigate the growth mechanism of the 3-D PTO nanostructure, different orientations of TiO-terminated STO substrates were prepared and reacted under the same hydrothermal synthesis conditions.26 Figure 2a,c,e shows the PTO nanostructures fabricated on (001), (110), and (111) oriented STO substrates, respectively. Remarkably, the orientation of the STO substrate clearly modulates the PTO morphology (Figure 2a,c,e), as 3-D nanostructures with crosshatched patterns were formed on (001) STO substrate, whereas linearly aligned nanostructures were formed on (110) and triangular pattern (indicated by red lines in Figure 2e) was produced on (111) oriented STO substrates. The PTO nanostructures on (001) STO were fabricated along the [100] and [010] directions, whereas in the case of (110) STO and (111) STO substrates, the nanostructures formed along the [100] direction of (110) STO and at 30° angle from [100] and [010] of (111) STO, respectively. An explanation for the growth tendency of PTO grains is shown through schematic surface atomic structure of STO substrates (Figure 2b,d,f) obtained via a rotation of the fundamental STO crystal structure. Because the STO (001), (110), and (111) surfaces are TiO-terminated, the alignment direction of the PTO

Figure 2. PbTiO3 nanostructures hydrothermally synthesized on (a) (001), (c) (110) and (e) (111) oriented SrTiO3 substrates. Surface atomic structures of TiO-terminated (b) (001), (d) (110) and (f) (111) oriented STO substrates. One unit cell thickness of surface atoms were projected. (g) Hierarchical 2-D PTO nanosheet network on TiO2/(001) STO Substrate. (h) Polycrystalline TiO2 thin film on (001) STO substrate annealed at 700 °C for 5 min.

nanostructure is closely related to the Ti and O atomic configurations on the substrate surface (see Figure 2b,d,f). The surface atomic structures of (001), (110), and (111) STO substrates have symmetry similar to the nanostructured arrangement of PTO crystals: square (4-fold rotational symmetry), linear (2-fold), and triangular (3-fold), respectively. Ti cations and O anions occupy corresponding sites in the STO and PTO lattices and they have similar symmetry and bonding environments. For this reason, Ti- and O-terminated surfaces of an STO substrate are likely to be effective nucleation sites for PTO nanostructure growth. The similarity in the symmetry of the nanostructures in Figure 2a,c,e with the symmetry of the arrangement of Ti and O in the STO substrate surfaces in Figure 2b,d,f suggests this is the case, and Ti and/or O atoms in the STO surface greatly influence the shape and direction of growing PTO crystals. Thus, we suggest the shape evolution and distribution of 3-D PTO nanostructures are directly related to the Ti and O arrangements on the surface of the STO substrate.27 To further test the role of arrangement of Ti−O atoms on the initial growth of 3-D PTO nanostructures, we deposited a randomly oriented TiO2 thin film on (001) oriented STO substrate using the sol−gel process. This film includes both Ti and O cations, but in completely different geometric arrangements from that of the underlying STO substrate. Figure 2h shows SEM images of the TiO2 sol−gel thin film spin-coated 2184

DOI: 10.1021/acs.chemmater.8b00135 Chem. Mater. 2018, 30, 2183−2187

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thin PTO film. The atomic ratio in Figure S2 was obtained from a large area of the image and is consistent with PbTiO3 and SrTiO3 stoichiometry. Figure 3b shows the Raman scattering spectrum of the 3-D PTO nanostructures recorded at room temperature (20 °C). All Raman modes of the 3-D nanostructure were indexed using earlier study of Burns and Scott.28 Most of the principal-axes modes were observed, shown in Figure 3b. The wavenumbers of Raman lines in the 3-D PTO nanostructure agree well with the previously reported results for single crystal29 and textured thin film PTO30 (see Table SI in the Supporting Information). The lowest frequency soft phonon mode (polar phonon mode), E(1TO), was below the detection limit of the equipment, but some signal was detected from another lowfrequency soft phonon mode, E(1LO). It is found that Raman modes E(1LO), A1(1TO), E(2TO), and E(3TO) are notably shifted to higher frequencies (relative to single crystal PTO), whereas the mode A1(2TO) is downshifted and the silent mode B1+E is upshifted slightly. This blue shift of Raman lines, which is unusual in nanostructured crystals, can be explained by assuming the presence of tensile stress in the specimen31 due to increased tetragonality of the 3-D PTO nanostructures. Figure 3c shows the temperature dependence of Raman spectra obtained from the 3-D PTO nanostructures near the Curie temperature. In the vicinity of the Curie point, the short- and long-range interactions compete with each other in such a way that the frequency of the transverse optical (TO) phonon approaches zero.32 With increasing temperature as shown in Figure 3c, the Raman lines shift to lower frequency and the peak width broadens significantly compared to the room temperature spectrum in Figure 3b. At 500 °C, one can still observe some prominent low-frequency peaks in the Raman spectra indicating the polar nature of the PTO nanostructures at this temperature.33 These results suggest that the 3-D PTO nanostructures could have a higher tetragonal-to-cubic phase transition temperature (Curie temperature) than that of bulk PTO (490 °C). In order to measure the piezoelectric property of PTO nanostructures, an electrically conducting (001) oriented, 0.5 wt % Nb-doped STO substrate was used in the hydrothermal synthesis as illustrated in Figure 3d inset. The local piezoelectric response of the PTO nanostructure was collected using piezoelectric response microscopy (PFM) under DC bias sweep from −12 to +12 V. The phase signal indicates that the PTO nanostructure exhibits unambiguous 180° ferroelectric domain switching behavior (Figure 3d). From the amplitude and piezoelectric response signals, it was found that the PTO nanostructure has a strong piezoelectric response (Figure 3e,f) compared to the bulk PTO. To further confirm the crystal structure of the PTO nanostructures on a (001) oriented STO substrate, we conducted microstructure analysis using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Figure 4a shows TEM image of a cross section through the PTO nanostructure grown on the (001) oriented STO substrate. The short PTO crystal in Figure 4a is 130 nm high and 240 nm wide. The PTO nanostructures are not continuous on the STO substrate, rather there are gaps between individual PTO crystals and the crystals have different thicknesses (see Figure 4a and inset). The growth shape of individual crystals was found to vary as evident from the regular rectangular shape depicted in Figure 4a and a more random shape in the inset of Figure 4a. The two white arrows in the

and annealed at 700 °C for 5 min. As shown in Figure 2h, the TiO2 thin film has a smooth surface with an average grain size of ca. 20 nm. Using this TiO2/STO substrate, the PTO nanostructure was hydrothermally synthesized under the same experimental condition used previously for the sample shown in Figure 2a,c,e. The PTO nanostructures grown on the randomly oriented TiO2 seed layer are depicted in Figure 2g. The presence of the polycrystalline TiO2 layer induces randomly aligned nanosheets of PTO rather than the highly organized 3D nanosheets of PTO formed on bare STO (Figure 2a). This result confirms that the geometric arrangement of Ti−O atoms on the substrate surface plays an important role in the directional growth of PTO nanostructures. The X-ray diffraction (XRD) patterns of 3-D PTO nanostructures on (001) oriented STO are shown in Figure 3a. Except for the peaks of the STO substrate, all the diffraction

Figure 3. (a) XRD patterns for the 3-D PTO nanostructure grown on (001) oriented STO substrate. (b) Raman scattering spectrum of 3-D PTO nanostructure at room temperature and (c) variation of Raman spectrum with temperature near the Curie temperature. (d) Amplitude and (e) phase from piezoelectric response taken from the crosshatched PTO 3-D nanostructures on (001) oriented Nb-doped STO substrates. (f) Combined signal of amplitude and phase of PTO 3-D nanostructures, which confirms the ferroelectric behavior of PTO nanostructure.

peaks can be indexed as tetragonal perovskite structured PTO (in agreement with positions given in JCPDS Card No. 772002). The XRD pattern also indicated that the 3-D nanostructures were fully grown along the c-axis direction. The composition of the 3-D PTO nanostructures with crosshatched pattern was analyzed using energy dispersive X-ray spectroscopy (EDS) mapping and is presented in Figure S2 (Supporting Information). These results clearly show the Pb and Ti atomic distribution in 2D nanosheets. Given the 1.0 μm depth of the EDS probe, the dark intercrystal regions between individual PTO nanosheets can arise from the STO substrate or 2185

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In summary, novel 3-D PTO nanostructures with aligned patterns were formed on TiO-terminated STO substrates using a hydrothermal synthesis process. Different crystalline patterns were obtained depending upon the STO substrate orientation; (001), (110), or (111) substrates produced cross-hatch, linear, and triangular patterns, respectively. The PTO nanostructures were aligned with symmetry consistent with that of Ti and O atomic configuration on the surface of STO substrate. When the STO substrate was coated with a thin film of polycrystalline TiO2 prior to hydrothermal synthesis, the resulting PTO nanostructures were random with no preferential alignment. 3D PTO nanostructures were single crystal plates of PTO grown along the (001) crystal direction. The Raman shift as a function of temperature near the phase transition temperature indicates that the 3-D PTO nanostructure has a Curie temperature above 500 °C. Using electrically conducting (001) oriented 0.5 wt % Nb-doped STO substrate, the piezoelectric response of the 3-D PTO nanostructure was confirmed. These 3-D PTO nanostructures with large surface area, high ferroelectric, and piezoelectric properties could be promising candidates for water splitting, nanogenerators, and high sensitivity strain sensors.

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Figure 4. (a) Cross-sectional TEM image for 2-D PTO nanosheets on (001) oriented STO substrates. Inset is TEM image of another region. (b) High-resolution TEM (HRTEM) image of the interface between STO substrate and PTO nanosheet. Inset is the SAED pattern taken from the area indicated by red square. (c) Magnified HRTEM image of the interface. For better visibility, the TEM image is artificially colored. (d) Scanning TEM (STEM) elemental mapping of Pb and Sr at the PbTiO3/SrTiO3 interface region.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b00135. Experimental details; SEM images of 3-D PTO nanostructures with cross-hatched patterns in different magnification; EDX analysis of PTO on (100) STO; schematic illustration of hydrothermal synthesis; autoclave for hydrothermal synthesis; frequencies of Raman modes in the 3-D PTO nanostructures at room temperature (PDF)

upper left of the figure indicate the location of ferroelectric domains. Figure 4b,c shows high-resolution TEM (HRTEM) images of the interface between STO substrate and PTO nanostructure. The lattice fringes of PTO nanostructure and the underlying STO substrate are in a cube−cube epitaxial relationship with the [001] PTO growth direction oriented perpendicular to the (001) substrate surface. The TEM results confirm that the individual PTO nanosheets are single crystal that grow along the [001] direction with respect to the STO substrate. This result is also in good agreement with the (001) textured XRD pattern of the PTO nanostructure depicted in Figure 3a. The selected area electron diffraction (SAED) pattern was taken from the area indicated by the red square in Figure 4b. The fundamental diffraction spots split into pairs; one-half for PTO and the other for STO phase because of the slightly different lattice parameters of the two phases (see the inset of Figure 4b). The brighter diffraction spots match well with the lattice spacing of PTO and the weaker spots are consistent with the lattice spacing of STO substrate. Figure 4c shows the magnified HRTEM image of the interface of the PTO nanostructure and the STO substrate. The spacing of the PTO plane, 3.917 Å, almost coincides with the 3.905 Å lattice spacing of (110) plane in STO, with only 0.3% lattice mismatch. Based on the d-spacing values measured from the diffraction pattern, the PTO nanostructure has higher tetragonality (c:a ratio of 1.070) than typical bulk crystals (c:a = 1.063); which is expected to result in higher polarization and Curie temperature.24,34,35 The result of the increased tetragonality is in line with the Raman blue shift and increased phase transition temperature of 3-D PTO nanostructures. As shown in Figure 4d, the line scan concentration profile across the interface obtained from STEM shows that Pb and Sr are strictly confined to the PTO and STO crystals, respectively.

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AUTHOR INFORMATION

Corresponding Author

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

Hyun-Cheol Song: 0000-0001-5563-9088 Min-Gyu Kang: 0000-0001-9247-6476 Author Contributions

H.-C.S. and M.S. conceived and designed the research. H.-C.S. performed the experiments and analyzed the data. M.-G.K. assisted the piezoelectric measurement by PFM. D.M. carried out TEM measurement with data analysis. R.J.B helped to measure Raman spectroscopy. W.T.R advised TEM data analysis and investigation of the growth mechanism of the nanostructure. S.P. and M.S. supervised the project. H.-C.S. wrote the paper, and all authors contributed to editing the paper. Funding

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), Air Force Office of Scientific Research (M.-G.S., grant # FA9550-14-1-0376), the Korea Institute of Science and Technology (H.-C.S., grant # 2E28210), and AMRDEC (H.-C.S.) Notes

The authors declare no competing financial interest. 2186

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DOI: 10.1021/acs.chemmater.8b00135 Chem. Mater. 2018, 30, 2183−2187