NbO3 Piezoelectric Nanofibers with Surface-induced Crystallization at

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Functional Nanostructured Materials (including low-D carbon)

Structure and High Performance of Lead-Free (K0.5Na0.5)NbO3 Piezoelectric Nanofibers with Surface-induced Crystallization at Lowered Temperature Yasmin Mohamed Yousry, Kui Yao, Xiaoli Tan, Ayman Mahmoud Mohamed, Yumei Wang, Shuting Chen, and Seeram Ramakrishna ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05898 • Publication Date (Web): 06 Jun 2019 Downloaded from http://pubs.acs.org on June 6, 2019

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ACS Applied Materials & Interfaces

Structure and High Performance of Lead-Free (K0.5Na0.5)NbO3 Piezoelectric Nanofibers with Surface-induced Crystallization at Lowered Temperature Yasmin Mohamed Yousry1,2, Kui Yao1, *, Xiaoli Tan3, Ayman Mahmoud Mohamed1, Yumei Wang2, Shuting Chen1, and Seeram Ramakrishna2

1 Institute

of Materials Research and Engineering, A*STAR (Agency for Science,

Technology and Research), 2 Fusionopolis Way, Innovis, 138634, Singapore 2 Department

of Mechanical Engineering, National University of Singapore, 9

Engineering Drive 1, 117575, Singapore 3 Department

of Materials Science and Engineering, Iowa State University, Ames, Iowa

50011, USA

* Corresponding author:

ACS Paragon Plus Environment

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E-mail: [email protected]

KEYWORDS: KNN, nanofibers, lead-free, piezoelectric, electrospinning

ABSTRACT: Lead-free potassium and sodium niobate (KNN) nanofiber webs with random and aligned configurations were prepared by electrospinning process from polymer-modified chemical solution. The crystallization process, structure, composition, dielectric, ferroelectric and piezoelectric properties of the nanofibers and nanofiber webs were investigated. Theoretical analysis and experimental results showed that the surfaceinduced heterogeneous nucleation resulted in the remarkable lower crystallization temperature for the KNN nanofibers with {100} orientation of the perovskite phase in contrast to the bulk KNN gel, and thus well-controlled chemical stoichiometry. Low dielectric loss, large electric polarization, and high piezoelectric performance were obtained in the nanofiber webs. In particular, the aligned nanofiber web exhibited further improved piezoelectric strain and voltage coefficients, and higher FOM than their thin film


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counterparts, promising for high performance electromechanical sensors and transducers applications.

Introduction


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Many emerging applications of sensors and transducers strongly demand flexible piezoelectric materials with highly efficient electromechanical coupling effect.1-3 Recently, one-dimensional (1D) piezoelectric nanostructures such as wires, rods, tubes, and fibers have been extensively studied because of their unique performance related to their high aspect ratio and anisotropic geometry.1-5 Given the strong dependency of piezoelectricity on the crystallographic orientation,6 current research in 1D piezoelectric nanostructures demonstrates new strategies for tuning material properties through control of geometry and utilization of the size effect.1,4,6 Anisotropic behaviours of crystal growth in geometrically constrained 1D polymeric nanostructures have been reported,7 and such behaviours have been utilized to enhance the piezoelectric performance of nanotubes for high frequency ultrasonic transducers.8 However, polymeric materials generally exhibit much lower piezoelectric properties than piezoelectric ceramics. Therefore, the study of 1D piezoelectric ceramic nanostructures is of great significance not only for fundamental understanding but also for device applications.


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Various methods have been utilized to fabricate 1D piezoelectric nanostructures, such as hydrothermal, solid-state reaction, molten salt, electrospinning, and template-based methods.1, 4, 9 Among these, the electrospinning process stands out as a simple, costeffective, and versatile technique to produce high quality 1D nanostructures.9 In recent years, it has been used to fabricate piezoelectric ceramic nanofibers.1,4,10-12 For example, Chen et al. reported that nanoscale lead zirconate titanate (PZT) fibers could be processed by a controlled sol–gel synthesis incorporated with the electrospinning process.10 In addition, electrospun PZT nanofibers have been reported to be used in piezoelectric nanogenerators.4,11,12

The potassium sodium niobate K0.5Na0.5NbO3 (KNN) ceramic is an excellent candidate of lead-free piezoelectric material with properties comparable to that of lead-containing piezoelectric ceramics.13 KNN crystalizes in the perovskite structure, possessing a moderate dielectric constant, high remanent polarizations, a low coercive field, high piezoelectric constants, and a high Curie temperature.14 However, the severe loss of alkali ions (K+, Na+) during processing has been known to lead to high leakage and


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negatively impacts the ferroelectric and piezoelectric properties.15-18 Therefore, thermal treatments at a reduced temperature are highly desired to minimize the volatility problems in KNN thin films. Moreover, improper annealing temperatures may result in the formation of secondary phases and poor electrical properties.16 To compensate their losses during the heat treatment process, excess amounts of K and Na are usually used for improving piezoelectric performance.17 In our previous works, we demonstrated that inclusion of excess K and Na in the precursor solution and proper selection of chemical agents are effective in reducing A-site vacancies in the perovskite structure, improving the degree of crystallinity, and enhancing the electrical properties of the solution-derived KNN films.1822

We further introduced doping elements and successfully produced 0.95(K, Na)(Sb,

Nb)O3-0.05(Bi, Na, K)ZrO3 thin films with outstanding piezoelectric properties.23,24 To explore scalable industry applications, we explored thermal spray processing for producing KNN-based piezoelectric ceramic coatings on stainless steel substrates.25

Compared to the research on KNN films, studies on KNN nanofibers are very limited due primarily to the difficulty in achieving high quality fibers. In 2012, Jalian and Girishin


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reported electrospun KNN fibers with an average diameter of 150 nm in randomly oriented webs, and characterized them in the super paraelectric state with large separations between nanofibers.26 In a follow up work, they observed strong anisotropy in the piezoelectric strain coefficient d33 with piezoelectric force microscopy (PFM), being 18.3 pm V-1 for on-axis fiber and 75.8 pm V-1 for out-of-axis fiber, respectively.27 Several additional reports attempted to reproduce the synthesis of high quality KNN nanofibers.2832

In these handful studies on KNN nanofibers, the crystallization mechanism and the

macroscopic piezoelectric properties remain unexploited. Hence, the present study intends to investigate the crystallization process of KNN nanofibers prepared by electrospinning from a solution with excess of alkali ions and stabilizing agent, and to evaluate the dielectric, ferroelectric, and piezoelectric properties over a macroscale on the nanofiber web. The product of the piezoelectric strain and voltage coefficients (d33 (eff),

g33 (eff)) of the un-doped KNN nanofiber web is used as a figure-of-merit (FOM) to evaluate the piezoelectric performance, which is shown to be superior to un-doped KNN thin films and comparable to un-doped PZT films. The high FOM and the understanding of the


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crystallization mechanism will inspire applications of electrospun KNN nanofibers in piezoelectric sensors and transducers.

Experimental Section

Materials: K0.5Na0.5NbO3 (KNN) precursor solutions with excess K and Na compositions (K:Na:Nb = 0.55:0.55:1) were prepared with 2-methoxyethanol (2-MOE) (anhydrous, 99.8%) as the solvent in dry nitrogen atmosphere. Potassium acetate (CH3OOK, A.C.S. reagent, 99+%), sodium acetate (CH3COONa, reagent, 99+%), and niobium ethoxide [(CH3CH2O)5Nb, anhydrous, 99.95%] were used as the starting chemicals. The solution was diluted with 2-MOE to obtain a concentration of 0.5 M, and then polyvinylpyrrolidone (PVP) powders (PVP360k) were added into the solution. The mole ratios of PVP to KNN were fixed at 1:1 (monomer: KNN).

Electrospinning deposition: The KNN solution was loaded in a 3-mL plastic syringe capped with a 21-gauge stainless steel needle (inner diameter = 0.82 mm). A high voltage of 15 kV was applied to the needle through a DC power supply (Gamma High Voltage, AU-30P1-L220V, Japan). The solution was injected from the needle at a constant rate of


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0.5 mL/h with a syringe pump (SP100IZ Syringe Pump, 789100W, U.S.A). A static collector made of a metallic plate covered with an Al foil was grounded and used as the nanofibers collector substrate for obtaining the randomly oriented nanofibers web. A rotating cylinder with a rotating speed of 1500 rpm was used for obtaining the aligned nanofibers web.

The wet nanofiber webs were dried at 150 °C for 3 min, followed by pre-pyrolysis at 350 °C for 3 min, post-pyrolysis at 450 °C for 5 min, and final annealing in furnace at different temperatures, 500 °C, 550 °C, 600 °C, 650 °C, and 700 °C, for 10 min with a heating rate of 8 ℃/min. A processing flow chart for preparing KNN nanofibers is summarized in Figure S1 (Supplementary Information). The thickness of the randomly oriented and aligned nanofiber webs was ~4 µm by controlling the deposition parameters of the electrospinning process. Patterns of Au top electrodes (0.25 mm diameter size) were deposited on the nanofiber webs by e-beam evaporator to serve for the electrical measurements. After Au electrodes were sputtered on top of the nanofiber webs, the webs were poled in thickness direction at room temperature under an electric field of 50 kV cm–1 for 3 min.


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Characterizations: The surface morphology was examined using a field emission scanning electron microscopy (FESEM, JSM-6700F, JEOL), combined with energydispersive spectroscopy (EDS). The crystalline structure analysis was performed at room temperature using an x-ray diffraction (Bruker 2D Micro XRD Cu K(alpha) system D8ADVANCE, Bruker AXS GmbH, Karlsruhe, Germany). High resolution transmission electron microscopy (HRTEM, TEM, JSM-6100F, JEOL, Ltd., Tokyo, Japan) and selected area electron diffraction (SAED) were carried out at 300 kV, where the nanofibers were dispersed in isopropyl alcohol (IPA) then a droplet of the nanofibers dispersion was put on a Cu microgrid supported with a carbon film. The thermal decomposition was studied by thermogravimetric analysis differential scanning calorimetry (TGA-DSC, SDT 2960, Simultaneous, TA Instrument, New Castle, DE). The volatilization of K and Na composition

during

heating

was

studied

by

thermogravimetric

analysis-mass

spectrometry (TGA-MS, STARe system, Mettler-Toledo, Schwerzenbach, Switzerland), in which the volatilized species from the thermogravimetric (TG) instrument were fed via a capillary to a mass spectrometer attached directly to the outgas port of the TG instrument. Dielectric properties were measured with an impedance analyzer (HP4294A,


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Agilent Technologies Inc., Tokyo, Japan). Polarization-electric field (P-E) hysteresis loops were characterized with a standard ferroelectric testing system (Premier II, Radiant Technologies, Inc., USA) at 10 Hz. The effective piezoelectric coefficient d33 (eff) of the nanofibers were measured by laser scanning vibrometer (LSV, OFV- 3001-SF6, PolyTech GmbH, Waldbronn, Germany) over a macroscopic area.

Results and discussion

Thermal characterization of the crystallization process

The randomly oriented and aligned KNN nanofiber webs were prepared using static and rotating collectors in the electrospinning process as shown in the schematic setup image in Figure S2 (Supplementary Information). The as-spun nanofibers contain a considerable amount of organic species. To convert them into piezoelectric KNN nanofibers, the organic components have to be removed and crystallites with the desired perovskite structure need to be formed during subsequent annealing. To reveal the


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decomposition of organic species and determine the proper annealing temperature, different

thermal

thermogravimetric

analysis

techniques

analysis-differential

were

employed.

scanning

Figure

calorimetry

1

shows

(TGA-DSC)

the

curves

measured on fibers from the aligned nanofiber web from room temperature to 800 °C in air. The precursor KNN gel was used as the control sample for comparison. The TGA data indicate a total mass loss of 81.72% for the gel, while 41.72% for the aligned nanofibers. The smaller weight loss for the nanofibers is due to the volatilization of the solvent during the deposition process of electrospinning. Three main mass loss regions are observed in the TGA curves. The first region reflects the loss of the residual solvent, the second region corresponds to the decomposition weight loss of organic species, and the third region is related to the crystallization of KNN to the perovskite structure. Interestingly, it is noticed that weight loss essentially stopped at 470 °C for the aligned nanofibers, in comparison to 525 °C for the KNN gel control sample. The DSC data, also shown in Figure 1, are consistent with the TGA results. The small endo/exothermic peaks between 270 °C and 370 °C are believed to be related to the formation and decomposition of the carboxylic and pyrochlore compounds before the


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formation of the final perovskite phase. The first exothermic peak at 418 °C in the DSC curves of both the aligned nanofibers and the gel is attributed to the decomposition of polyvinylpyrrolidone (PVP). The strong exothermic peaks at 472 °C and 520 °C on the DSC curves of the aligned nanofibers and the gel, respectively, are attributed to combustion of organic constituents and crystallization of KNN. They correlate very well to the completion of the weight loss revealed by TGA. The large amount of heat due to the combustion of the added PVP could locally self-heat the nanofibers and hence promote the crystallization of the nanofibers at a lower nominal temperature. The promotion of the crystallization of perovskite phase by the exothermic organic combustion has previously been reported in the potassium sodium niobate and lead magnesium niobate systems.18, 33

The results indicate that while the temperature for full crystallization completion is

570 °C for the KNN gel, which corresponds to the exothermic peak evident in the DSC curve without corresponding weight loss in the TGA curve, it is remarkably reduced by 98 °C, to 472 °C for the aligned KNN nanofibers. It should be noted that a lower crystallization temperature is also observed in nanofibers from the random nanofiber web, in comparison with the same precursor gel, as presented in the TGA-DSC curves in Figure


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S3 (Supplementary Information). The TGA-DSC data clearly demonstrate that the unique 1D geometry of the nanofibers can significantly reduce the crystallization temperature and promise for enhancing the crystallinity of KNN as shown below.

Figure 1. TGA-DSC curves of aligned KNN nanofibers and precursor gel. The samples were heated in air from room temperature to 800 °C, at a rate of 10 °C/min.

To further clarify the chemical decomposition process during heating of the as-spun fibers, the thermogravimetric analysis-mass spectrometry (TGA-MS) was conducted, with results shown in Figure 2. For KNN precursor gel, as shown in Figure 2(a), substantial loss of K and Na occurred between ~300 °C to 620 °C, as indicated from NaO and KO curves. In contrast, the loss of K and Na in KNN nanofibers was reduced significantly and occurred between ~300 °C to 530 °C, as shown in NaO and KO curves of Figure


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2(b). This is primarily due to the significantly reduced crystallization temperature in KNN nanofibers. In addition, for KNN nanofibers, the peaks of the NaO, Na2O, KO, K2O curves appear simultaneously with the second peak of the NH3 and C4H7 curves at 430 °C, which is associated to the total decomposition of PVP. 18, 33 This is in contrast to the gel sample where those peaks appear at different temperatures. Our explanation is that the heat due to PVP combustion effectively promoted crystallization of perovskite phase in KNN nanofibers and thus the Na and K loss could be suppressed once perovskite phase formed, but this did not happen in the gel as crystallization of perovskite phase only happened at higher temperature. Thus, the lower crystallization temperature in KNN nanofibers pushed the peaks of the KO and NaO loss peaks to lower temperatures, in comparison to the KNN gel. Therefore, due to the unique 1D geometry of nanofibers and the addition of PVP, chemical loss of Na and K was significantly suppressed and the formation of perovskite phase was promoted at lower temperatures.


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Figure 2. TGA-MS curves of (a) KNN precursor gel, and (b) aligned KNN nanofibers. The samples were heated from room temperature to 800 °C, at a rate of 10 °C/min in N2.

Crystalline phase formation during thermal treatment Based on the thermal analysis results, the as-spun KNN nanofiber webs were annealed at a series of temperatures (450, 500, 550, 600, 650, and 700 °C). The development of the crystalline phase was analyzed with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The setup for the XRD experiments is schematically illustrated in Figure S4 (Supplementary Information). Figure 3(a) and (b) shows the XRD patterns of the random and aligned nanofiber webs, respectively. It is observed that when annealed at 450 °C, both nanofiber webs are still amorphous and crystallization has yet to start. After annealing at 500 °C, strong diffraction peaks appear, indicating the main perovskite


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phase with a secondary pyrochlore phase (identified as K4Nb6O17, PDF # 00-014-0287). The XRD results are in agreement with the crystallization temperature determined by the thermal analysis. At higher annealing temperatures of 550 °C, 600 °C, and 650 °C, both nanofiber webs exhibit a single perovskite phase with gradually improving crystallinity. The crystallinity is further improved at 700 °C, accompanied with an apparently intensified {100} diffraction peaks. However, a trace of secondary phase is observed at 700 °C, which is isostructural with NaNbO3 and could be resulted from the more severe loss of K at the high temperature. The strong {100} diffraction peaks imply the existence of preferred orientation of the crystallites in the KNN nanofibers. To quantify the degree of {100} orientation preference, the ratio of the intensity of the {100} diffraction peaks to those of {110} is used, which is 0.68 in KNN ceramic powders with no preferred orientation. For the aligned nanofiber web, the intensity ratio is 0.94 at 550 °C, 1.17 at 600 °C, 1.40 at 650 °C, and 4.1 at 700 °C. The ratio is slightly lower at each temperature for the random nanofiber webs. This indicates a strong preference of {100} orientation preference in crystallites developed in KNN nanofibers, particularly in nanofibers annealed at higher temperatures. Such


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preferred {100} orientation was also observed in KNN thin films.34 This is due to the lower surface energy of {100} in KNN compared to other crystallographic planes,35 acting as the favoured to growth direction during crystallization. It should be mentioned that the XRD experiments were done on two sample configurations with 90° apart, as shown in Figure S4 (Supplementary Information). No difference in XRD results was observed between these configurations, indicating that the preferred {100} planes are along the transverse radial direction of the annealed KNN nanofibers, as schematically represented in Figure 3(c). Moreover, the average crystallite size in the KNN nanofibers was calculated using K∗ λ

Scherrer’s equation: D = B ∗ cos 𝜃, where D is the crystallite size, λ is the wavelength of Xray radiation (0.15406 nm), B is the full width at half maximum (FWHM) of the most intense diffraction peak, θ is the angle of diffraction, k is a constant (having the value 0.89).36 The calculation results are presented in Figure 3(d) for both the random and aligned nanofiber webs. The crystallite sizes of both nanofiber webs increase with increasing temperature, indicating a crystal growth process. In addition, the larger


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crystallite size of the aligned nanofiber webs indicates a higher overall crystallinity than the random nanofiber webs.

Figure 3. XRD patterns of KNN nanofibers annealed at different temperatures 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C for (a) randomly oriented and (b) aligned nanofiber webs. (c) Schematic representation for the orientation of (100) planes located at the surfaces and along the radial direction of the annealed KNN nanofibers. (d) Crystallite size of the randomly oriented and aligned KNN nanofibers as a function of the annealing temperature.


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To directly visualize the morphology and reveal the crystal structure of the nanocrystallites in annealed KNN nanofibers, transmission electron microscopy (TEM) was performed on the nanofibers from aligned nanofiber webs. Figures 4(a) and 4(b) show the bright field micrographs while Figure 4(c) displays the selected area electron diffraction (SAED) pattern of the KNN nanofibers annealed at 600 °C. Similarly, Figures 4(d), (e), (f) are TEM results on nanofibers annealed at 650 °C and Figures 4(g), (h), (i) are the results at 700 °C. It is evident that KNN nanofibers annealed at 600 °C are in polycrystalline form and the crystallites appear to be in a cubic form with a size around 20 nm, which is in good agreement with the crystallite size determined by XRD. The SAED pattern in Figure 4(c) is acquired along the zone-axis from the region highlighted in Figure 4(b). The main set of diffraction spots are strong and sharp, with additional spots from surrounding crystallites. Upon increasing the annealing temperature to 650 °C, the cubic shape of the crystallites becomes more obvious and their size increased to about 65 nm, as shown in Figure 4(d) and (e). The SAED pattern shown in Figure 4(f) acquired from the region marked in Figure 4(e) is along the zone-axis with sharp diffraction


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spots. At a higher annealing temperature of 700 °C, significant grain growth took place. The crystallites occupy the whole cross-section of the nanofiber and their size reached to ~115 nm, as shown in Figure 4(g), and (h). Furthermore, KNN nanofibers start to break down into nanosized crystal cubes, leading to a structure collapse of the nanofiber configuration. This appears to be in sharp contrast to KNN thin films that still preserve their morphology even after annealing at 700 °C. 20 The collapse of the fibrous structure and the formation of the nanosized single crystals are in support of the XRD results where the highest degree of preferred {100} orientation is detected as presented in Figure 3(b). The SAED pattern shown in Figure 4(i) was acquired along the zone-axis, confirming that the KNN nanofiber annealed at 700 °C is composed of single crystals along its length. Interestingly, the SAED pattern exhibits ½{oeo}-type superlattice spots (marked with a red circle; o: odd Miller indices; e: even Miller indices). It should be noted that our KNN composition is the polymorphic phase boundary composition and is supposed to be a mixture of orthorhombic (Amm2) and tetragonal (P4mm) phases, both of which do not exhibit any superlattice diffraction spots.37,38 The appearance of the ½{oeo} superlattice diffraction spots were reported previously in doped KNN ceramics,


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and was interpreted as the presence of a lower symmetry monoclinic Pm phase.39 In our KNN nanofibers annealed at 700 °C, the existence of the Pm phase is likely related to formation of the second phase as revealed by the XRD data shown in Figure 3(b). The more severe loss of K at 700 °C leads to oxygen octahedral tilting in the perovskite structure, resulting in the ½{oeo} superlattice spots in SAED.


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Figure 4. (a) and (b) TEM micrographs of aligned KNN nanofibers annealed at 600 °C, (c) SAED pattern acquired from the marked area in (b) along the zone-axis. (d) and (e) TEM micrographs of aligned KNN nanofibers annealed at 650 °C, (f) SAED pattern acquired from the marked area in (e) along the zone-axis. (g) and (h) TEM micrographs of aligned KNN nanofibers annealed at 700 °C, (i) SAED pattern recorded from the marked area in (h) along the zone-axis. One of the ½{oeo}-type superlattice spots is highlighted with the red circle.


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Theoretical considerations on the crystallization process With the atoms at a free surface or an interface experience a dissimilar local environment and lower coordination compared with those in the bulk of a material, the energy associated with these atoms and their equilibrium position is significantly higher.40 As the size of materials reduces to the nanoscale, the influence of the created free surfaces in nanofibers is substantially enhanced in comparison to the bulk due to the increased surface-to-volume ratio, as illustrated in Figure 5(a). Crystallization of KNN nanofibers derived from chemical solution deposition involves nucleation from amorphous phase and grain growth processes, and both are dependent on the crystalline surface energy. Given the enormous surface-to-volume ratio, the energy associated with the atoms of the nanofibers will be higher compared to that of the bulk. Hence, the more surfaces reduce the energy barrier from Ea to Ea* for crystallization of nanofibers compared to bulk, as represented in Figure 5(b). A theoretical analysis on the faster nucleation rate in KNN nanofibers than the bulk is provided in Supplementary Information. The high rate surface-induced heterogeneous nucleation with the reduction in the energy


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barrier contributed to the lower crystallization temperature of KNN nanofibers than the bulk gel (Figures 1 and 2), and the polycrystalline nature (Figures 3 and 4).

Figure 5. (a) Schematic illustration for the surface effects in nanofibers configuration compared to the bulk counterpart. (b) Comparison of energy barrier required for the nucleation of nanofibers and bulk, where Ea and Ea* are the activation energy barrier for bulk material and nanofibers, respectively, and ΔG is the Gibbs free energy.

Morphology of crystalized KNN nanofiber webs Figure 6(a) and (b) show the field emission scanning electron microscopy (FESEM) micrographs of the KNN nanofiber web annealed at 600 °C with randomly oriented nanofibers, while Figure 6(d) and (e) are for the aligned nanofiber web. The micrographs


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clearly indicate that the nanofibers collected by the static collector are essentially disordered and randomly oriented, while those on the rotating collector are uniaxially aligned. Prior to annealing, the as-spun nanofibers are smooth, dense, and have a larger diameter of ~200 nm, as shown in Figure S5 (Supplementary Information). After thermal treatment at 600 °C, the diameter of the nanofibers significantly decreases to ~50-100 nm due to the removal of the organic components. The dimensional shrinkage is much larger than bulk gel, and the nanofibers are much more dense than bulk gel after annealing. Figure 6(c) and (f) shows the representative energy disperse X-ray spectrum (EDS) of the annealed random and aligned KNN nanofiber webs, respectively. The Al signal is from the collector bottom electrode. Elemental analyses of the EDS spectra indicate that the random and aligned nanofiber webs consist of Na: K: Nb with atomic ratios of 0.48: 0.50: 1 and 0.50: 0.51: 1, respectively, indicating a good stoichiometric control achieved with nanofibers due to the significantly lowered crystallization temperature. Close examination reveals that the intensity of the energy peaks from the aligned nanofiber web is higher than that of the random nanofiber web due to a higher packing density in the former, which could be one of the reasons for the higher crystallinity


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as observed for the former. No obvious delamination was observed from the crosssectional FESEM micrographs of the annealed KNN nanofiber webs, as shown in Figure S6 (Supplementary Information).

Figure 6. FESEM micrographs of KNN nanofiber webs annealed at 600 °C for (a) and (b) the random nanofiber web at low and high magnifications, (d) and (e) the aligned nanofiber web at low and high magnifications, respectively. (c) and (f) Representative EDS spectra for the random and aligned nanofiber webs, respectively.

Dielectric and ferroelectric properties of KNN nanofiber webs The high crystallinity, preferred {100} orientations, suppressed loss of alkali ions in KNN nanofibers with the lowered crystallization temperature, and their high packing density in


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the nanofiber webs all point to possible high electrical properties. Dielectric and ferroelectric properties on a macroscopic scale (using 0.25 mm top electrodes, electrodes configuration and the polarization direction are shown in Figure S7 (Supplementary Information)) were measured for KNN nanofiber webs annealed at 600 °C. Figure 7(a) presents the dielectric properties as a function of frequency at room temperature measured from both types of webs. The dielectric constant from the random nanofiber web, 440 at 1 kHz, is lower than 497 from the align nanofiber web, probably because the random nanofiber web has a lower packing density. It should be noted that the dielectric loss for the random and aligned webs are very low, as 0.032 and 0.023, respectively, which may be attributed to the good chemical stoichiometric control. The electric polarization - field loops of the KNN nanofiber webs annealed at 600 °C are presented in Figure 7(b), with unsaturated polarization observed under applied field up to 70 kV/cm. Further increasing electric field led to large leakage although the dielectric loss is small at low field. After carefully measuring the electrode area, a large remanent polarization Pr of 22.7 µC cm-2 is observed with apparent coercive field Ec of 23.8 kV cm-1 for the aligned nanofiber web. It is noted that the remanent polarization is significantly


28

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larger while the coercive field is much lower than that of KNN thin films even without polarization saturation.20,21 The difference is believed primarily due to the geometry and mechanical clamping of the samples. The thin film samples are solid with sharp surface and subjected to the substrate clamping. In contrast, for the KNN nanofiber webs, the interfacial area between the electrode and the irregular fabric sample are substantially larger than the nominal electrode coverage area, and the mechanical clamping effect applied onto the nanofibers is significantly reduced due to the nature of the 1D fiber structure. These two factors result in much larger observed remanent polarization and smaller coercive field for the nanofiber webs than thin films. With the denser package and improved crystallinity and {100} orientation, the aligned nanofiber web exhibit smaller leakage, larger remanent polarization, and smaller coercive field in Figure 7, than the random nanofiber web.


29


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Figure 7. (a) Frequency dependence of dielectric constant and loss, and (b) the ferroelectric P-E hysteresis loops of the randomly oriented and aligned KNN nanofiber webs annealed at 600 °C, measured at room temperature.

Piezoelectric properties of KNN nanofiber webs To assess the potential of the KNN nanofiber webs for piezoelectric device applications, macroscopic piezoelectric properties are evaluated using a laser scanning vibrometer (LSV), which is a reliable and efficient method in determining the piezoelectric longitudinal strain coefficient.41 During the test, a unipolar AC signal of 5 V in amplitude at 1 kHz was applied to the nanofiber web with top Au electrode. Figure 8(a) and (b) is a threedimensional presentation of the vibration data of the random and aligned nanofiber webs, respectively, under the electric driving. The central protruding portion is the electrically


30

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excited area under the top electrode, and the surrounding flat region is the KNN nanofiber web not covered by the top electrode. As the displacement profile of the top electrode area is not flat, the average displacement values were used in determining the dilatation or strain of the KNN nanofiber webs. The effective piezoelectric strain coefficients (d33 (eff)) of the random nanofiber web is determined to be 75 pm V-1, while that of the aligned one is 98 pm V-1. It should be highlighted that the d33

(eff)

value in the present work is the

highest ever reported in KNN nanofibers. Apparently, the well-controlled chemical stoichiometry, high crystallinity with {100} orientation, and mitigated substrate clamping for the KNN nanofibers, particularly in the aligned nanofiber web, contributed to the outstanding piezoelectric properties. Before this work, only local piezoelectric d33 characterized by PFM, with the maximum value of 75 pm V-1 was reported in the literature. 27, 28, 31


31


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Figure 8. Three-dimensional graph of vibration data of the KNN nanofibers under the electric sine wave driving (5 V amplitude at 1 kHz), measured with LSV under substrate clamping condition for (a) randomly oriented and (b) aligned KNN nanofiber webs. The central protruded area is the electrically excited nanofibers under the top Au electrode (diameter size of 0.25 mm), whereas the flat adjacent area is the nanofibers without the top electrode.

With the dielectric constant values from Figure 7(a), the corresponding effective piezoelectric voltage coefficient g33 (eff) can be calculated, as 19.2 and 22.1 mm V N−1 for the random and aligned nanofiber webs, respectively. For many electromechanical sensors and transducers including mechanical energy harvesters, both high piezoelectric strain and voltage coefficients (d33 (eff), g33 (eff)) are sought in the meantime. As such, the product of these two piezoelectric coefficients is often used as a figure of merit (FOM),


32

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FOM = 𝑑33 (𝑒𝑓𝑓).𝑔33 (𝑒𝑓𝑓) = 𝑑233 (𝑒𝑓𝑓)/𝜀𝑜𝜀𝑟.The piezoelectric properties of our KNN nanofiber webs are summarized in Table 1, in comparison with those of KNN and PZT thin films in the literature. It is evident that a superior FOM is achieved in the aligned and un-doped KNN nanofiber web in the present work, compared to un-doped KNN thin films,18,22,42 and comparable with un-doped PZT thin films.41,43,44 Such an excellent piezoelectric performance makes the KNN nanofibers very attractive for electromechanical device applications. Table 1. Piezoelectric and dielectric properties of KNN nanofibers, in comparison with the reported macroscale properties of un-doped KNN and PZT thin films measured at room temperature and 1 kHz. 18,22,41-44

Random KNN

Aligned KNN

KNN Thin

PZT Thin

Nanofibers

Nanofibers

Film 18,22,42

Film 41,43,44

d33 (eff) (pm V-1)

75

98

56-78

102-117

εr

440

497

680-800

650-750

g33 (eff) (mm V N-1)

19.2

22.1

9.3-16.6

15.3-20.3

FOM (µm2 N-1)

1.4

2.2

0.5-1.2

1.6-2.4

Conclusions


33


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KNN nanofiber webs with random and aligned configurations were prepared by electrospinning process from a PVP-modified solution. It was found that perovskite phase with preferred {100} orientation along the radial direction formed at significantly lowered crystallization temperature of 472 °C in the KNN nanofibers compared to 570 °C for the KNN bulk gel, and thus effectively suppressed loss of alkali ions, leading to a wellcontrolled chemical stoichiometry. The analysis indicated that the high surface-to-volume ratio and thus reduced energy barrier due to surface-induced heterogeneous nucleation resulted in the lower crystallization temperature for the KNN nanofibers. The effective composition control achieved with lowered crystallization temperature, high crystallinity with {100} orientation, and mitigated substrate clamping with the nanofiber package structure contributed to low dielectric loss, large electric polarization, and high piezoelectric performance. In particular, with the further improved package density and {100} orientation preference, the aligned nanofiber web exhibited further increased piezoelectric strain and voltage coefficients, d33 (eff) of 98 pm V-1, and g33 (eff) of 22.1 mm V N−1, respectively, thus produced a large 𝑑33 (𝑒𝑓𝑓).𝑔33 (𝑒𝑓𝑓) as a FOM superior to the undoped KNN thin films and even comparable to un-doped PZT thin films. Therefore, our


34

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obtained

electrospun

KNN

nanofibers

are

propmising

for

high

performance

electromechanical sensors and transducers applications. ASSOCIATED CONTENT Supporting Information A processing flow chart for preparing KNN nanofibers (Figure S1), schematic images of the electrospinning process with static and rotating collectors (Figure S2), TGA-DSC curves of random oriented KNN nanofibers and precursor gel (Figure S3), configuration of the X-ray diffraction experiment before and after 90 ° rotation (Figure S4), FESEM of as-spun randomly oriented and aligned KNN nanofiber webs (Figure S5), crosssectional FESEM images for randomly oriented and aligned KNN nanofiber webs (Figure S6), and electrode configuration and the polarization direction for the nanofiber web (Figure S7).

AUTHOR INFORMATION

Corresponding Author * K. Yao. E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The work was designed and planned by K. Yao, Y. M. Yousry and S. Ramakrishna. X. Tan provided analyses and suggestion of TEM. Material processing and characterization experiments were done by Y. M. Yousry. K. Yao, Y. M. Yousry, A. M. Mohamed, S. Chen, Y. Wang, and S. Ramakrishna did the analyses and discussion of the results. ACKNOWLEDGMENT The authors acknowledge the research grant support in part by Singapore Maritime Institute under the Maritime Sustainability R&D Programme, Project ID: SMI-2015-MA07 (IMRE/16-7P1125). REFERENCES (1) Liang, L.; Kang, X.; Sang, Y.; Liu, H. One‐Dimensional Ferroelectric Nanostructures: Synthesis, Properties, and Applications. Adv. Sci. 2016, 3, 1500358.


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Table of Contents


45


Figure 1. TGA-DSC curves of aligned KNN nanofibers and precursor gel. The samples were heated in air from room temperature to 800 °C, at a rate of 10 °C/min. 108x82mm (220 x 220 DPI)


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Figure 2. TGA-MS curves of (a) KNN precursor gel, and (b) aligned KNN nanofibers. The samples were heated from room temperature to 800 °C, at a rate of 10 °C/min in N2. 166x67mm (220 x 220 DPI)



Figure 3. XRD patterns of KNN nanofibers annealed at different temperatures 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C for (a) randomly oriented and (b) aligned nanofiber webs. (c) Schematic representation for the orientation of (100) planes located at the surfaces and along the radial direction of the annealed KNN nanofibers. (d) Crystallite size of the randomly oriented and aligned KNN nanofibers as a function of the annealing temperature. 164x126mm (220 x 220 DPI)


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Figure 4. (a) and (b) TEM micrographs of aligned KNN nanofibers annealed at 600 °C, (c) SAED pattern acquired from the marked area in (b) along the zone-axis. (d) and (e) TEM micrographs of aligned KNN nanofibers annealed at 650 °C, (f) SAED pattern acquired from the marked area in (e) along the zone-axis. (g) and (h) TEM micrographs of aligned KNN nanofibers annealed at 700 °C, (i) SAED pattern recorded from the marked area in (h) along the zone-axis. One of the ½{oeo}-type superlattice spots is highlighted with the red circle. 266x238mm (300 x 300 DPI)



Figure 5. (a) Schematic illustration for the surface effects in nanofibers configuration compared to the bulk counterpart. (b) Comparison of energy barrier required for the nucleation of nanofibers and bulk, where Ea and Ea* are the activation energy barrier for bulk material and nanofibers, respectively, and ΔG is the Gibbs free energy. 94x104mm (220 x 220 DPI)


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Figure 6. FESEM micrographs of KNN nanofiber webs annealed at 600 °C for (a) and (b) the random nanofiber web at low and high magnifications, (d) and (e) the aligned nanofiber web at low and high magnifications, respectively. (c) and (f) Representative EDS spectra for the random and aligned nanofiber webs, respectively. 167x87mm (220 x 220 DPI)



Figure 7. (a) Frequency dependence of dielectric constant and loss, and (b) the ferroelectric P-E hysteresis loops of the randomly oriented and aligned KNN nanofiber webs annealed at 600 °C, measured at room temperature. 172x65mm (220 x 220 DPI)


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Figure 8. Three-dimensional graph of vibration data of the KNN nanofibers under the electric sine wave driving (5 V amplitude at 1 kHz), measured with LSV under substrate clamping condition for (a) randomly oriented and (b) aligned KNN nanofiber webs. The central protruded area is the electrically excited nanofibers under the top Au electrode (diameter size of 0.25 mm), whereas the flat adjacent area is the nanofibers without the top electrode. 154x70mm (220 x 220 DPI)



Table of Contents 258x139mm (300 x 300 DPI)


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NbO3 Piezoelectric Nanofibers with Surface-induced Crystallization at

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