Approaching piezoelectric response of Pb-piezoelectrics in

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Functional Nanostructured Materials (including low-D carbon)

Approaching piezoelectric response of Pb-piezoelectrics in hydrothermally synthesized Bi (Na K) TiO nanotubes 0.5

1-x

x

0.5

3

Mohammad Bagher Ghasemian, Aditya Rawal, Yun Liu, and Danyang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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ACS Applied Materials & Interfaces

Approaching piezoelectric response of Pbpiezoelectrics in hydrothermally synthesized Bi0.5(Na1-xKx)0.5TiO3 nanotubes Mohammad Bagher Ghasemian a, Aditya Rawal b, Yun Liu c and Danyang Wang a* a

School of Materials Science and Engineering, UNSW Sydney, Sydney, NSW 2052, Australia

b

Nuclear Magnetic Resonance Facility, Mark Wainwright Analytical Centre, UNSW Sydney, Sydney, NSW 2052, Australia c

Research School of Chemistry, The Australian National University, ACT 0200, Australia

Keywords: Bi0.5(Na1-xKx)0.5TiO3, one-dimensional, nanotube, lead-free, piezoelectric

ABSTRACT: A large piezoelectric coefficient of 76 pm/V along the diameter direction, approaching that of lead-based piezoelectrics is observed in hydrothermally synthesized Pb-free Bi0.5(Na0.8K0.2)0.5TiO3 nanotubes. The 30-50 nm diameter nanotubes are formed through a scrolling and wrapping mechanism without the need of a surfactant or template. A molar ratio of KOH/NaOH=0.5 for the mineralizers yields the Na:K ratio of ~0.8:0.2 corresponding to a orthorhombic-tetragonal phase boundary composition. XRD patterns along with TEM analysis

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ascertain the coexistence of orthorhombic and tetragonal phases with (110) and (001) orientations along the nanotube length direction, respectively. 23Na NMR spectroscopy confirms the higher degree of disorder in BNKT nanotubes with O/T phase coexistence. These findings present a significant advance towards the application of Pb-free piezoelectric materials.

1. INTRODUCTION Piezoelectric materials are extensively employed in many applications including sensors, actuators, ultrasonic generators etc. Despite their superior properties, the highly toxic components arising from Pb[ZrₓTi₁₋ₓ]O₃ (PZT) based piezoelectric materials largely restricted their applications in environmentally-friendly products. Novel fabrication approaches play a key role in global efforts to find out the lead-free replacements which possess comparable piezoelectric properties to those of PZT. Promising lead-free candidates include (Bi0.5Na0.5)TiO3 (BNT), (Bi0.5K0.5)TiO3 (BKT), BaTiO3 (BT) and (K, Na)NbO3 (KNN).1,2 Bi-based perovskite oxides are non-toxic and have a stereochemically active lone pair of 6s2 electrons leading to a high polarizability and structural distortion, often resulting in large polarizations.3 In spite of its high Curie temperature (TC), strong remanent polarization and large piezoelectric coefficient, BNT has greatly suffered from its high coercive field, causing difficulty in poling BNT. Nevertheless, this issue can be alleviated easily by site engineering through alloying BNT with various elements, resulting in enhanced functional properties.4–6 Many high-performance piezoelectric materials exhibit a transition region in their composition phase diagrams, known as a morphotropic phase boundary (MPB), where the crystal structure alters abruptly and the electromechanical characteristics are maximum.7 Among BNT-based systems, Bi0.5(Na1-xKx)0.5TiO3 (abbreviated as BNKT) exhibits a rhombohedral-tetragonal MPB in the composition range x = 0.16–0.20 8 associated with excellent piezoelectric properties. The


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optimal dielectric and piezoelectric properties for [Bi0.5(Na1-xKx)0.5]TiO3 have been reported at x=0.18.9,10 While many BNKT thin films

11–13

and bulk ceramics 14–16 have been reported, recently a new

class of functionalized one-dimensional (1D) morphologies (or generally low dimensional systems) such as nanotubes and nanofibers which demonstrate excellent figure of merits for piezoelectric and thermoelectric applications, are gaining significant interest. These superior properties arise from their large surface to volume ratio

17,18

of the 1D materials, in comparison

with their bulk counterparts, in addition to possessing a unique density of electronic state leading to higher magnetic, optical and electrical properties.19,20 Particularly, 1D piezoelectric materials ideal for energy harvesting owing to their high mechanical strength and sensitivity to mechanical movements.21 Tubular structures have larger internal surface area and high adsorption microcavities compared to fiber geometry.20 For example, piezoelectric nanotubes can be employed to release drugs into the patient tissue or act individually to carry the small amount of ink in jet printing, in addition to applications such as actuators, sensors and data-storage.22 The results obtained from a core-surface model indicated that as-grown nanotubes are able to generate a significantly higher piezoelectric potential than their nanofiber counterparts due to the geometric properties of nanotubes, especially, this geometry effect becomes more remarkable in nanotubes with smaller radius or smaller wall thickness-to-radius ratio.18 Many methods are employed for synthesizing 1D piezoelectric materials including sol-gel template, molten salt, solid-state reaction, electrospinning, hydrothermal etc.17,23 PbZr0.52Ti0.48O3, PbTiO3, BaTiO3 and SrTiO3 nanotubes were synthesized using the template-assisted technique 21 while a TiO2 nanotube conversion through a hydrothermal treatment also has been reported to produce these perovskite nanotubes.24 In addition, ZnO 25 and Sr0.8Bi2.2Ta2O9


26

nanotube arrays

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were grown on a GaN substrate and inside a porous Si template, respectively, through a lowtemperature solution chemical way. Synthesis of piezoelectric materials by conventional solidstate technique usually needs a high-temperature calcination process and often results in large grains. In Bi-containing systems, high sintering temperature causes Bi evaporation and formation of defects leading to a high leakage current in these materials. The impediments for wide application of electrospinning technique include the use of polymers or organic solvents, high energy consumption, low production rate and process complexity. Hydrothermal method outperforms these techniques because of the lower treatment temperature, direct fabrication without the need for substrates, higher reaction and nucleation rate and lower cost.27 Specifically, its low treatment temperature effectively suppresses the undesired fluctuations in stoichiometry induced by the loss of volatile elements in piezoelectric materials. While the need of environmentally-friendly piezoelectric materials seems inevitable, the production of site-engineered BNT-based materials through the green, cost-effective and highly productive methods alleviates the issue significantly. The modified [Bi0.5(Na1-xKx)0.5]TiO3 system with the specific 1D nanostructure is suggested as a highly-competent alternative of PZT, especially at MPB composition. In this work, Bi0.5(Na0.79K0.21)0.5TiO3 nanotubes were selforganized through a facile hydrothermal method. Different ratios of mineralizers were examined to obtain tubelike morphology with the phase boundary composition in the resultant BNKT nanostructures. Structural and piezoelectric properties of the as-grown nanotubes as a function of increasing K+ incorporation were also characterized and compared to parent BNT to decouple the influence of morphology and structure on the piezoelectric response. The phase co-existence and hollow structure of the hydrothermally-synthesized BNKT nanotubes significantly improved their piezoelectric properties which are comparable to those of 1D lead-based piezoelectric


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materials, indicating the potential of our environmentally-friendly BNKT nanotubes as a promising alternative to Pb-based piezoelectrics. 2. EXPERIMENTAL Analytical grade sodium and potassium hydroxides were used as the sodium and potassium sources, respectively, as well as mineralizers to foster the formation the solid phase through dissolution-precipitation processes in the hydrothermal reaction. In order to obtain MPB-like composition x = 0.18 in BNKT, various KOH/NaOH ratios of 0.2, 0.5 and 1 were used in the precursors and the final products were assigned to BNKT1, BNKT2 and BNKT3, respectively. The flowchart in Figure 1 describes the hydrothermal process of synthesizing BNKT nanostructures. We adopted the optimized conditions for synthesizing BNT-based piezoelectric nanofibers in our previous work.28 First, 30 ml of a 12 M solution of mineralizers was prepared. Then, stoichiometric amounts of BiCl3 and TiO2 (1:2 mol/L) were added to this solution. The obtained solution was transferred to an autoclave and stirred for one hour before the hydrothermal treatment in an oven at 200 ˚C for 20 hours. Finally, after cooling the autoclave naturally to the room temperature, the slurry solution was filtered and as-synthesized products were washed with distilled water to natural pH and dried completely at 80 ˚C overnight. The phase structures of the as-synthesized BNKT nanostructures were characterized by X-ray diffraction (Cu-Kα radiation, λ= 1.54 Å, Philips X’Pert Pro MPD). Morphology of nanostructures

were

imaged

by

scanning

electron

microscopy

(SEM,

Nova

NanoSEM450). A Bruker energy dispersive spectroscopy (EDS) was employed to determine the chemical composition of the specimens. The evolution of phase structure as a function of K+ concentration was verified by a Raman spectrometer (inVia Reinshaw) at room temperature with an Ar-ion laser excitation source (continuous mode; 514 nm) in


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the backscattering geometry of 180°. High-resolution transmission electron microscopy (HR-TEM, Phillips CM200) and selected area electron diffraction (SAED) patterns were used to further confirm the dimensions, crystallite size and lattice structure of the nanotubes. Solid-state

23

Na nuclear magnetic resonance (NMR) spectroscopy (Bruker

Avance III 700 spectrometer) investigations were carried out for the

23

Na Nuclei,

operating at a frequency of 185 MHz to study the effect of doping and composition modification on the lattice structure. BNKT powders were packed in 4 mm zirconia rotors and spun to 14 kHz at the magic angle. A 2 μs hard pulse was used to acquire 1D

23

Na

NMR spectra and the 23Na NMR peak of sodium chloride (NaCl) was referenced at 0 ppm to measure chemical shifts. Triple- Progress in engineering high tum pulse sequence with an excitation pulse of 8.5 μs and a conversion pulse of 2.5 μs with a 20 μs z-filter followed by a 60 μs selective 90° pulse for detection were applied to achieve the 2D MQMAS (multiple-quantum magic-angle spinning) NMR spectra. Spectral deconvolution was analyzed by the Dmfit software.29 Piezoelectric properties of the 1D BNKT nanostructures were measured using a piezoelectric force microscopy (PFM, Asylum Research), equipped with a Pt-coated silicon tip. The piezoelectric response was obtained using a force contact of 3 N/m with DC and AC voltages of ± 30 V and 1 V, respectively. 3. RESULTS AND DISCUSSION Figure 2 shows the comparative XRD patterns of BNT, BNKT1, BNKT2 and BNKT3 nanostructures. The crystal structure of all products shows a perovskite phase. BNT presents a highly crystallized single-phase structure which has been indexed according to an orthorhombic lattice structure as it was proved in our previous study on hydrothermally-synthesized BNT nanofibers.28 BNKT samples comprised secondary phases such as potassium and sodium


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titanates as evidenced by the extra peaks (marked by the diamond symbols ◊ and) in the XRD patterns. BNKT1 shows a very small peak splitting at (111) peak indicating the slight change in BNT lattice structure by doping potassium. However, by increasing the KOH/NaOH ratio in the precursors, (111) peak doublets can be seen, which implies K+ fully diffused into the BNT lattice creating a homogenous BNKT solid solution, because neither orthorhombic BNT nor tetragonal BKT show (111) peak splitting. In BNKT2, the intense peak (110) loses the symmetric shape and becomes broader at the left side which is the clue of the coexistence of T-phase and O-phase, implying that BNKT2 has moved to O-T phase boundary. Similarly, a peak splitting of (200) into (200)T and (002)T and (211) into (211)T and (112)T indicate a tetragonal (T) symmetry which is more evident in BNKT3.30 XRD peaks of BNKT show a minor shift to lower angles compared with those of BNT due to the substitution of larger K+ (1.33 Å) for Na+ (1.02 Å) in A-site. Accordingly, this peak shift will be more significant by increasing the amount of K+ cations in BNKT structure.31 Figure 3a and 3b show the refined unit cell volumes and lattice parameters of BNKT nanostructures based on the orthorhombic and tetragonal structures, respectively. It is clearly seen a gradual expansion in lattice constants and unites volume with the increase of K+ concentration. As indicated by the XRD patterns, a substantial change in BNKT composition occurred by increasing the KOH concentration in mineralizer solution. Figure 4 exhibits the EDS spectra of BNKT samples. Table 1 demonstrates the atomic percentages of elements in BNKT obtained from the EDS analysis. To compare the composition change more easily, they are normalized by Ti atomic percentage. A continuous increase in K+ content can be seen from BNKT1 to BNKT3. Based on the XRD and EDS results, BNKT2 possesses a composition in the vicinity of an O-T


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phase boundary (x = ~0.20). It is concluded that atomic ratios in nanofibers are different from their ratios in precursors, indicating the Na+ and K+ competition in filling the A site of BNKT, especially at higher concentrations of OH-.32 1D BNKT materials are created through an initial growth and ripening mechanism. With a sufficiently high mineralizer concentration, starting materials hydrolyze during the hydrothermal process and the produced crystallites accumulate together at a planar interface and a similar crystallographic orientation to create nanofibers. Thermodynamically, reducing the surface free energy through the elimination of the pairs of high energy surfaces is the main driving force for this naturally oriented attachment.33 The chemical reactions below describe the creation of BNKT materials in our hydrothermal process:34 12 Bi3+ + Ti4+ + 40 OH- → Bi12TiO20 + 20 H2O (1) Bi12TiO20 + 12(1-x) Na+ + 12x K+ + 23 Ti4+ + 104 OH- → 24 Bi0.5(Na1-xKx)0.5 + 52 H2O (2) In the hydrothermal reaction, precursors hydrolyse into Bi2O3.nH2O and TiO2.nH2O and subsequently dissolved in aqueous media to Bi3+ and Ti(OH)x4-x, respectively. However, the solubility of TiO2.nH2O is much lower than Bi2O3.nH2O which hinders the dissolutioncrystallization mechanism. At the early stage of the hydrothermal reaction, Bi2O3.nH2O and TiO2.nH2O accumulate together to minimize the surface free energy.35 When the concentration of mineralizers (NaOH and KOH) and temperature are relatively high, the dissolving rate of TiO2.nH2O to Ti(OH)x4-x increases and accelerates the kinetics of the dissolutionrecrystallization process. Then dissolved Bi3+, Na+, K+ and Ti(OH)x4-x are dispersed in the hydrothermal solution and BNKT structures are precipitated directly from the homogeneous solution.36


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Figure 5 exhibits the SEM images of BNKT nanostructures with different compositions. All BNKTs have a 1D morphology with uniform diameter size distribution. BNKT1 nanofibers demonstrate an average diameter of ~150 nm, the diameter of nanofibers decreases dramatically to less than 50 nm in BNKT2 and BNKT3. Two factors that affect the size of 1D BNKTs include disorder degree (randomness) of the crystal structure and Na+/K+ radii difference. In general, the highly disordered system is not favorable for crystallization.37 The degree of disorder in a doped crystal is usually higher than that of its parent phase, thus hindering the grain growth.30 Secondly, as the ionic radius increases significantly from Na+ to K+ (1.02 Å to 1.33 Å), the substitution of K+ for Na+ in BNT lattice enlarges the unit cell size and causes a slight deformation of BNT structure. This prevents the further growth of BNT particles and results in thinner BNKTs nanofibers. However, these effects are negligible in BNKT1 nanofibres due to the lower concentration of K+ ions as confirmed by EDS results (Table 1). In addition, it is known that secondary phases may greatly affect the grain growth of materials during the hydrothermal process. Since BNKT2 and BNKT3 1D nanostructures contained trace secondary phases, their diameters are significantly smaller than that of the BNKT1 nanofibers. Figure 6 shows the room temperature Raman spectra of hydrothermally-synthesized 1D BNKT materials. The Raman bands are relatively wide due to the overlapping of Raman modes and Asite disorder.38 A1(TO) bands at ~130 cm-1, ~270 cm-1 and ~550 cm-1 are assigned to Na-O, Ti-O and TiO6 octahedral vibrations in perovskite structures, respectively.39 Raman spectra of BNKT are almost the same as that of BNT. Since bismuth cation (mBi = 208.98) is much heavier than potassium (mK = 39.10) and sodium (mNa = 22.99) cations, the mass effect for most of the modes involving these two alkali elements is negligible. However, the significant difference in ionic radii between Na+ (rNa+ = 1.02 Å) and K+ (rK+ = 1.33 Å) causes structural framework and


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polyhedron natural distortion by increasing K+ concentration in BNT lattice.40 The Na-O vibration mode decreased in intensity and slightly shifted to lower frequencies. Since the position of this band has altered with the potassium concentration, the band at ~130 cm-1 can be attributed to Na/K-O vibrations indicating the mass growth on A site.41 The broad band between 450-700 cm-1 splits into two distinguishable peaks from BNKT1 to BNKT3 which illustrates an evolution in phonon behavior caused by ion replacement in A-site. This split is more visible in BNKT3. The nanofiber nature of BNKT1 nanostructures were confirmed by transmission electron microscopy (TEM) images as revealed in Figure S1 (Supporting Information). Figure 7a shows a bright field TEM image of a bunch of 1D BNKT2 and a morphology of nanotubes is clearly identified. Similarly, BNKT3 nanostructures exhibited a nanotube morphology as shown in Figure S2 (Supporting Information). Figure 7b and 7c display the HRTEM images of a single BNKT2 nanotube. BNKT nanotubes can be up to 0.5 µm long with a nearly constant outer diameter of ~30 nm. The nanotubes are open from both sides and the inner diameter varies between 5-10 nm, as shown in Figure S3 (Supporting Information). Some minor amount of nonwrapped multilayered nanosheets can be observed and are marked in Figure 7a. BNKT nanotubes exhibit a multi-wall morphology with a symmetrical and identical number of walls with a distance of ~0.72 nm between successive layers as shown in Figure 7c. The formation of nanotubes, is understood on the basis of the growth of the crystalline threedimensional TiO2 structure. The TiO2 three-dimensional framework is built when TiO6 octahedra share vertices edge. However, a high concentration of the alkaline solution in hydrothermal reaction breaks some of Ti-O-Ti bonds of the titanium precursor and motivates the creation of layered titanates in the form of thin sheets composed of octahedra TiO6 units charged balanced by alkali metal ions. Under high temperature and pressure of hydrothermal process, these titanate


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sheets are exfoliated to one or two-layer nanosheets before rolling into nanotubes. The driving force for rolling and curving of nanosheets possibly arises from the asymmetry caused by the doping of the nanosheets with sodium/potassium or hydrogen and unsymmetrical surface forces caused by the locally high surface energy.42 Since in chemical compounds with a layered structure, the interaction energy between atoms of the same layer is higher than that between atoms of adjacent layers,42,43 the growth of nanosheets at the edge of individual layers is preferred on the initiation of a new layer.44 Simultaneously, the width of different layers may vary during the spontaneous crystallization and fast growth of layers which creates an imbalance in the layer width and motivates layers to move and curve within the multi-walled nanosheets in order to reduce the excess surface energy.42 The rolling process of nanosheets to nanotubes happens with slow growth rates due to the high concentration of mineralizers. The complete rolling of nanosheets is difficult and often results in the bending of these nanosheets at the edge (Figure 7b).44 As shown in SEM images, the increase of KOH concentration to NaOH reduces the growth rate and subsequently the size of 1D BNKT structures.30,37 This factor also acts as driving force of changing morphology from BNKT1 nanofiber to BNKT2 nanotube. Selected area electron diffractions (SAED) TEM image of BNKT1 and BNKT3 around the [11-1] and [010] zone axes are exhibited in Figure 8a and 8b, respectively. The SAED patterns confirm the orthorhombic and tetragonal structures of BNKT1 and BNKT3, respectively. The orthorhombic phase in BNKT1 nanofibers is consistent with structure of BNT nanofibers in our previous work.28 However, the SAED pattern of BNKT2 nanostructure, as shown in Figure 8c, exhibits both orthorhombic (Green) and tetragonal (Red) phases simultaneously, indicating the coexistence of O-T transition phases. To clarify the coexistence of orthorhombic and tetragonal phases, the inverted colour SAED patterns of BNKT2 nanotubes with schematic patterns of the


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two phases were provided in Figure S4 of Supporting Information. HR-TEM image in Figure 8d displays the highly crystalline nature of BNKT2 nanofibers. Two different interplanar spacing of 0.27 nm and 0.39 nm respectively assigned to (112) and (001) planes of orthorhombic and tetragonal phases were discerned in the HR-TEM image. The growth directions along [112] and [001] were identified for orthorhombic and tetragonal structures, respectively. Solid-state

23

Na NMR spectroscopy was employed to understand the effect of composition

modification on lattice structure of these lead-free materials. Previous work

45

on 23Na NMR of

sintered BNBT systems demonstrated that the NMR is sensitive to the disorder in the A-site which is a result of the subtle changes in the tilt of the TiO6 octahedra. Figure 9 shows 1D 23Na NMR spectra of the BNKT1D materials. The perovskite A-site disorder in materials such as BNT nanofibers (~200 nm diameter), results in significantly broadened seen in Figure 9a. Analogously, the BNKT1 yields

23

23

Na NMR spectra as

Na NMR spectrum with similar peak

broadening also, but with additional resolved resonances superimposed on the main peak, which are attributed to the formation of Na2Ti3O7 phase in the material. In comparison with BNKT1, the full width at half maximum (FWHM) of

23

Na NMR signal peak increased in BNKT2 and

then reduces back again in BNKT3. It is expected the increase in FWHM of NMR peak is associated with the increased disorder in such materials. However, considering the results of the XRD and TEM measurements, in the present case the increased FWHM of the 23Na NMR signal peak in BNKT2 is attributed to the coexistence of O and T phase, i.e. disorder in the A site is present in both phases. In this context, tetragonal BNKT3 yields a narrower 23Na NMR signal. It is interesting to note that the similar peak broadening in BNT, BNKT1 and BNKT3 indicates that both the tetragonal and orthorhombic phases have a similar degree of A-site disorder. Previous work on BNBT systems,46 has assigned the

23

Na NMR signal broadening to the effect of a


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unimodal distribution of octahedral tilting. The increase in the K+ content also leads to a monotonic peak shifting from -11.8 ppm in BNKT1 to -13.5 ppm in BNKT3. These shifts echo the change in the lattice structure of these materials. The shoulders at -8.4 ppm and -7.2 ppm for BNKT2 and BNKT3, shoulders, respectively, were assigned to Na2Ti9O19 phase as evidenced in the XRD pattern. To get a deeper understanding of the effects of the K+ doping,

23

NA MQMAS experiments

were carried out to resolve the chemical and quadrupolar induced shifts. Figure 10 shows the MQMAS measurement results. It is clear that the 1D BNKT materials have distinct

23

Na

environments which are evident from the differences in their second-order quadrupolar interactions as seen in the 1D spectra extracted from the 2D MQMAS data. These 1D slices were deconvoluted with a combination of the Gaussian isotropic model (GIM) fits and second-order quadrupolar powder patterns. The details of the fits are presented in Table S1 (Supporting Information). In BNKT, sodium environment is assigned as “A” showing a broad chemical shift distribution, which is consistent with A-site disorder. However, an additional disordered site “B” with a smaller chemical shift distribution, but a stronger quadrupolar coupling is also evident. This site is not well resolved in BNKT2 while it appeared in BNKT3 with higher intensity. Sites “C” and “D” are present in all the BNKT 1D materials, and the lineshape simulations show that these sites have discrete values for the quadrupolar interaction, and yield conventional secondorder quadrupolar lineshape. In particular, these sites are specifically well resolved in BNKT3. At this point, the origin of sites “C” and “D” can be attributed to the presence of trace amount of Na-containing non-perovskite structured impurities, such as Na2Ti9O19. Piezoelectric properties of 1D BNKT structures with different compositions were measured by a piezoelectric force microscope with dual AC resonance tracking (DART) mode and a Pt-coated


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silicon tip. The 1D materials were placed on a silicon wafer as described in the previous work.28 The local piezoelectric response of BNKT nanofibers/tubes was characterized along the diameter direction as shown schematically in Figure 11a. Figures 11b, 11c and 11d, show topography, piezoresponse amplitude and piezoresponse phase images of a 150 nm-diameter BNKT1 nanofiber, respectively. Phase-voltage hysteresis loops and out-of-plane piezoresponse amplitude-voltage butterfly loops of 1D BNKTs with different compositions are shown in Figure 11e and 11f, respectively. The 180˚ phase reversal and hysteretic behaviour of 1D BNKTs ascertained their ferroelectric nature. The slope of linear part of amplitude-voltage butterfly loop was used to estimate the piezoelectric coefficient (d33). The average d33 of 20 pm/V was achieved for BNKT1 nanofibres. This is slightly higher than that of BNT nanofibers (d33= 15 pm/V)

28

The piezoelectric coefficients of BNKT2 nanotubes showed a significant rise to 76 pm/V. This substantial improvement in d33 of BNKT2 nanotubes is attributed to its chemical composition at O-T phase boundary. Nanotubes with the coexistence of O and T phases present much larger d33 than nanofibers which possess orthorhombic phase only. The lower anisotropy energy between mixed phases facilitates the motion of domain walls. Both mechanisms assist the polarization switching through poling and improve piezoelectric properties of our BNKT2 nanotubes. The hollow structure of nanotubes is mechanically more flexible and suffers from the less clamping effect on the piezoelectric response, leading to a substantial improvement in the piezoelectric coefficient of BNKT2 nanotubes. However, d33 decreased to 39 pm/V in BNKT3 nanotubes because of its off-O/T composition with x = ~0.35. Figure S5 shows the PFM amplitude and phase curves of the silicon wafer were measured as a reference. Given the fact that no obvious piezoelectric activity can be detected from the Si, it is concluded the observed piezoresponse in Figure 11f arises from the BNKT 1D nanostructures.


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Compared with BNKT1, in addition to the desired nanotube structure, anisotropy energy in BNKT3 reduces as θ decreases from orthorhombic to the tetragonal structure as described in Figure 12.47 The spontaneous polarization (Ps) directions of orthorhombic BNKT1 and tetragonal BNKT3 are [110] and [001], respectively. When a DC voltage is applied to a nanofiber along the diameter direction, the dipoles in T-phase are more prone to rotate toward the applied electric field direction, thus giving rise to a higher d33 value of 39 pm/V in tetragonal BNKT3 nanotubes. The obvious voltage offset in the phase hysteresis and amplitude butterfly loops of 1D BNKTs is presumably due to the flexoelectric effect.48 The presence of different structures and orientations in 1D BNKT materials create nonuniform strain fields or strain gradients and induces a built-in electric field, leading to the voltage offset.49 Figure 13 compares the piezoelectric coefficients of some typical 1D piezoelectric materials synthesized by a hydrothermal technique. Our BNKT nanotubes at the O-T phase boundary show one of the best d33 among the lead-free candidates (Blue).17,28,45,50–56 It is even higher or comparable to those some lead-based piezoelectric materials (Red)

57,58

such as PbZr0.2Ti0.8O3,57

suggesting the high potential of our BNKT nanotubes in the applications of next generation of nanogenerators. 4. CONCLUSION In this work, a competent lead-free 1D piezoelectric material has been fabricated through a cost-effective and highly productive hydrothermal method. Bi0.5(Na0.8K0.2)0.5TiO3 nanotubes with co-existence of O and T phases were synthesized hydrothermally by adjusting the ratio of mineralizers (KOH/NaOH=0.5) without the need of any template or surfactant. A scrolling and wrapping of multilayered nanosheets mechanism is suggested for the formation of BNKT nanotubes. KOH played an essential role as K+ supplier and mineralizer, and K+ concentration is


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the driving force to motivate the curving and wrapping of multilayered nanosheets through reducing the growth rate and inducing imbalance in layers width. Bi0.5(Na0.79K0.21)0.5TiO3 nanotubes rendered a higher degree of disorder on A-site due to the coexistence of O and T phases rather than single phase in off-O-T compositions as confirmed by

23

Na NMR

spectroscopy. The BNKT nanotubes presented a superior piezoelectric response of 76 pm/V over the other hydrothermally-synthesized 1D piezoelectric materials which is attributed to the facile domain wall motions with O-T composition and more mechanical flexibility of the nanotube structures.


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ASSOCIATED CONTENT Supporting Information TEM images of BNKT1 nanofibers, TEM images of BNKT3 nanotubes, HR-TEM image showing the inner diameter and interplanar distance of a BNKT2 nanotube, Inverted-color SAED patterns of BNKT2 nanotubes, Piezoresponse of a silicon substrate, and fitting parameters MQMAS NMR analyses.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

ORCID Mohammad Bagher Ghasemian: 0000-0002-5618-0106 Aditya Rawal: 0000-0002-5396-1265 Yun Liu: 0000-0002-5404-3909 Danyang Wang: 0000-0002-7883-8001 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was partially supported by the Australian Research Council Discovery Project (Grant No. DP170103514). A. Rawal acknowledges the Mark Wainwright Analytical Centre at UNSW for access to the solid-state NMR spectrometers funded through ARC LIEF (Grant No. LE0989541).


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Figure 1. Hydrothermal process of synthesising 1D BNKT nanostructures with different concentrations of mineralizers.

Figure 2. Comparative XRD patterns of BNT and BNKT nanostructures with different concentrations of potassium.


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Figure 3. Lattice constants and unit cell volumes of BNKT materials as a function of composition. a) orthorhombic and b) tetragonal structure.

Figure 4. EDS spectra of BNKT nanostructures with different compositions.


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Figure 5. SEM images of hydrothermally-synthesized 1D BNKT with different compositions: a) BNKT1, b) BNKT2 and c) BNKT3.


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Figure 6. Raman spectra of 1D BNKT materials measured at room temperature.


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Figure 7. Bright field TEM image of a) a bunch of 1D BNKT2, b) and c) a single BNKT2 nanotube.


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Figure 8. SAED patterns of a) BNKT1, b) BNKT3 and c) BNKT2, and d) high-resolution TEM image of the BNKT2 nanotube.


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Figure 9. Solid-state

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Na MAS NMR of a) BNT

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(Reproduced by permission of The Royal

Society of Chemistry), b) BNKT1, c) BNKT2 and d) BNKT3 1D nanostructures.


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Figure 10. Solid-state

23

Na MQMAS NMR of hydrothermally-synthesized a) BNKT1, b)

BNKT2 and c) BNKT3 1D materials. The 1D spectra extracted in the MAS dimension along with fits to the 1D spectra are shown on the left side of each figure, respectively. The specific chemical shift where each of the 1D spectra was extracted is indicated by the dashed line.


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Figure 11. PFM results of 1D Bi0.5(Na1-xKx)0.5TiO3: a) radial direction of a nanofiber against the PFM tip on substrate

56

(Published by The Royal Society of Chemistry), b) topography, c)

amplitude, d) phase image of BNKT1 nanofibres, e) piezoresponse phase hysteresis loops and f) piezoresponse amplitude butterfly loops of 1D BNKTs with different compositions.


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Figure 12. Change in spontaneous polarization direction through lattice distortion from orthorhombic to tetragonal.

Figure 13. Comparison of the piezoelectric coefficients of some one-dimensional nanostructures synthesized by hydrothermal method (References are on top of each column).


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Table 1. Elemental composition of hydrothermally-synthesized BNKT nanostructures with different mineralizer ratios.

KOH/NaOH

Normal At% Bi

Na

Normal At% / Ti At% Approximate K

Ti

Bi

Na

K

Ti composition

BNKT1 0.2

22.09 21.28 1.25 55.37 0.4

0.38 0.02 1 Bi0.5(Na0.95K0.05)0.5TiOx

BNKT2 0.5

19.11 15.37 4.29 61.22 0.31 0.25 0.07 1 Bi0.5(Na0.79K0.21)0.5TiOx

BNKT3 1

19.36 14.09 7.5

59.06 0.33 0.24 0.13 1 Bi0.5(Na0.65K0.35)0.5TiOx


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Table of Contents (TOC)


37

Approaching piezoelectric response of Pb-piezoelectrics in

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