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Synthesis of Monoclinic Potassium Niobate Nanowires that are Stable at Room Temperature Seungwook Kim, Ju-Hyuck Lee, Jaeyeon Lee, Sang-Woo Kim, Myung Hwa Kim, Sungnam Park, Haegeun Chung, Yong-Il Kim, and Woong Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja308209m • Publication Date (Web): 12 Dec 2012 Downloaded from http://pubs.acs.org on December 16, 2012
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Synthesis of Monoclinic Potassium Niobate Nanowires that are Stable at Room Temperature Seungwook Kim†, Ju-Hyuck Lee⊥, Jaeyeon Lee‡, Sang-Woo Kim⊥, Myung Hwa Kim‡, Sungnam Park|, Haegeun Chung||, Yong-Il Kim§,* and Woong Kim†,&,* †
Department of Nano-Semiconductor Engineering, |Department of Chemistry, &Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea ⊥
School of Advanced Materials Science and Engineering, SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea
‡
Department of Chemistry & Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea
||
Department of Environmental Engineering, Konkuk University, Seoul 143-701, Republic of Korea Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea
§
Supporting Information Placeholder ABSTRACT: We report synthesis of potassium niobate (KNbO3) nanowires with a monoclinic phase, a phase not observed from bulk KNbO3 materials. The monoclinic nanowires can be synthesized via a hydrothermal method using metallic niobium as a precursor. The nanowires are meta-stable, and thermal treatment at ~450 oC changed the monoclinic phase into orthorhombic phase which is the most stable phase of KNbO3 at room temperature. Furthermore, we fabricated energy-harvesting nanogenerators by vertically aligning the nanowires on strontium titanate substrates. Monoclinic nanowires showed significantly better energy conversion characteristics than orthorhombic nanowires. Moreover, the frequency doubling efficiency of monoclinic nanowires was ~3 times higher than that of orthorhombic nanowires. Our demonstration may contribute to the synthesis of materials with new crystalline structures and hence improving properties of the materials for various applications.
Alkaline niobates including KNbO3 are receiving increasing attention due to their excellent nonlinear optical, piezoelectric, ferroelectric, and photocatalytic properties.1 For example, KNbO3 is a promising material for optical applications such as optical wave guiding, frequency doubling, and holographic storage.2 Also, alkaline niobates are prime candidates for lead-free and environmentally-friendly piezoelectric materials to replace currently dominant lead-zirconium-titanates.3 As a nanowire form, the applications of alkaline niobates can be greatly extended to the emerging research fields of nanogeneratorbased energy harvesting and nanobiotechnology. For example, Suyal et al. demonstrated that KNbO3-based nanowires can be useful elements in nanometric electromechanical devices by measuring piezoelectric response and polarization switching.4 On the other hand, Nakayama et al. demonstrated the potential of KNbO3 nanowires as tunable nonlinear optical probes of scanning microscopy for physical and biological sciences.5
Nanowires can have novel characteristics distinguished from those of bulk counterpart owing to their low dimension, and this may provide new opportunities for research and technological development.6-13 For example, it has been recently reported that hydrothermally grown KNbO3 nanowires have a monoclinic phase that has not been observed from their bulk counterpart.14 This is remarkable because the low symmetry phase typically generates phenomenal dielectric and electromechanical responses. However, the monoclinic phase of KNbO3 nanowires was observed only at low temperature (T ~ 80 K) so far, which impedes investigations on and applications of the materials. Therefore, developing new synthetic methods to produce low symmetry phase of given materials that are stable at room temperature could be a critical breakthrough for both fundamental research and technological development. In this work, we report a novel synthetic route to monoclinic KNbO3 nanowires that are stable at room temperature. The space group (P1m1), lattice parameters, and atomic positions of the monoclinic nanowires were determined by X-ray and neutron diffraction experiments and Rietveld analysis. Heat treatment of the nanowires at 450 oC led to the conversion of the monoclinic phase to orthorhombic phase. Moreover, we demonstrated that vertically aligned KNbO3 nanowires can be grown on latticematched strontium titanate (SrTiO3) substrates. Vertically aligned monoclinic nanowires showed ~4 times higher power generation than orthorhombic nanowires. Additionally, frequency doubling by second-harmonic generation was observed from the nanowires upon laser excitation. Our demonstration of a robust synthetic protocol to a stable low-symmetry phase of KNbO3 nanowires may add a new perspective, especially in the study of piezoelectric materials. The KNbO3 nanowires were synthesized via a hydrothermal reaction using metallic niobium (Nb) powder. So far, the most widely used route to the synthesis of KNbO3 nanowires has been the hydrothermal method using niobium oxide (Nb2O5) powder as a precursor.5,14-17 However, the nanowires produced from this method have an orthorhombic phase at room temperature. Moreover, in spite of the popular use, this method requires ~1 week of
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Figure 1. (a) A 2 ml-vial containing KNbO3 nanowires. (b) Scanning electron microscope (SEM) image of KNbO3 nanowires (inset: high magnification SEM image, scale bar = 300 nm). (c) X-ray diffraction (XRD) patterns of the nanowires after heat treatment at various temperatures (ref. data is from JCPDS 771098), (d) zoom-in XRD patterns in the range of 44o to 46o, (e) Raman spectra of the nanowires in the range of (e) 150 to 950 and (f) 180 to 400 cm-1. reaction time.16,17 In our work, we found that Nb was more reactive than Nb2O5 in hydrothermal reaction, through which monoclinic KNbO3 nanowires were successfully synthesized only in 12 hours. In addition, our method produces KNbO3 nanowires in a gram-scale. Figure 1a shows a vial of the KNbO3 nanowire powder produced by Nb-based hydrothermal reaction. Scanning electron microscope (SEM, Hitachi S-4800) image clearly shows that the white powder consists mostly of KNbO3 nanowires (Figure 1b). The inset of Figure 1b shows that individual nanowires have square-shaped cross-sections. The thickness and length of the nanowires were 74.0 ± 11.7 nm (average ± one standard deviation, number of samples = 100) and 5.1 ± 1.4 µm, respectively. For the synthesis of KNbO3 nanowires, metallic Nb powder (Alfa Aesar) and potassium hydroxide (KOH, Sigma-Aldrich) were used as Nb and K source, respectively. KOH aqueous solution (15 M) was obtained by dissolving 12.624 g of KOH in 15 ml of deionized water followed by ultrasonication. Subsequently, 0.874 g of metallic Nb powder (~0.63 M) was added to the KOH solution in a 30 ml Teflon lined stainless steel autoclave. The autoclave was heated to 150 oC, after 12 hours of reaction, it was naturally cooled down to room temperature. Nanowires in white powder form were obtained by rinsing the product with deionized water and precipitating it with 5-min centrifugation at 2000 rpm. This rinsing step was repeated until the pH of the solution reached ~7. Finally, the sample was dried at 80 oC overnight. The mass of the final product was > 1.2 g, indicating that approximately 70% of the Nb source was incorporated into the KNbO3 nanowires.
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The KNbO3 nanowires have a monoclinic phase and are metastable at room temperature. Their crystalline structures were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500V/PC, Cu Kα radiation) and Raman spectroscopy (Renishaw inVia) (Figure 1c-f). As-grown nanowires showed a stable monoclinic phase as indicated by XRD patterns and Raman spectra. Our results are consistent with the data of monoclinic KNbO3 nanowires observed at low temperature as recently reported by Louis et al. and are clearly different from those of any phases of bulk KNbO3.14 Especially, three peaks observed near 45 degree in XRD pattern and sharp peak near 280 cm-1 in Raman spectrum are clearly different from those of orthorhombic phase (Figure 1cf). While the monoclinic phase was previously observed at significantly low temperature (~80 K),14 a monoclinic phase that is stable at room-temperature was obtained in this work for the first time. In general, bulk KNbO3 materials show multiple phase transitions as temperature changes; cubic to tetragonal phase at 435 o C, tetragonal to orthorhombic phase at 225 oC, and orthorhombic to rhombohedral phase at -10 oC.18 Therefore, the orthorhombic phase is the most stable phase of bulk KNbO3 at room temperature. Consistently, our hydrothermally grown nanowires showed orthorhombic phase after they were heat-treated at 450 oC. Heattreatment was carried out for 30 min in air and the sample was naturally cooled down to room temperature. Furthermore, we determined the crystal structure of the monoclinic KNbO3 nanowires by neutron and X-ray diffraction and Rietveld analysis. As a result, P1m1 space group was assigned to the structure. The lattice parameters were determined to be a = 4.04976(6), b = 3.99218(6), c = 4.02076(7) Å , and β = 90.1012(27)o. Combined Rietveld refinement was carried out using General Structure Analysis System (GSAS) (Rwp = 5.6%, Rp = 3.92%, Re =3.70%, S = Rwp/Re = 1.23, detailed information is presented in Experimental, Figure S1, S2, S3 and Table S1 in supporting information (SI)). Our results clearly verify the monoclinic phase of the nanowires. Although it is not completely clear why our synthetic conditions generate nanowires of monoclinic phase more preferably than those of orthorhombic phase, we suspect that use of a more reactive metallic Nb precursor instead of Nb2O5 could be critical. The Nb precursor, compared to Nb2O5 precursor, generates KNbO3 nanowires in shorter reaction time (12 hours vs. 6 days). High reactivity of the precursor could lead to a kinetically favored monoclinic phase owing to lower activation energy. On the other hand, thermodynamically favored orthorhombic phase could be obtained if sufficiently high energy is applied, e.g., in the form of heat. Indeed, we observed that metastable monoclinic nanowires were transformed into thermodynamically stable orthorhombic nanowires upon thermal treatment at ~450 oC. Consistently, it has also been widely reported that various materials including oxides with metastable crystal phases can be kinetically prepared with reactive precursors19,20,21 To determine the crystal structure, the X-ray diffraction was carried out over the scattering angle range of 20o ≤ 2θ ≤ 145o at a 2θ step of 0.02o using CoKα radiations with a graphite monochromator in the reflection geometry (Rigaku, Dmax 2200 V). The neutron powder diffraction was performed over scattering angles between 0o and 160o with a 2θ step of 0.05o using 1.8343 Å on the high resolution powder diffractometer at the Hanaro Center of the Korea Atomic Energy Research Institute (KAERI). Individual KNbO3 nanowires were single crystalline and showed lattice parameters consistent with those obtained from the Rietveld refinement. Crystal structures of the individual nanowires were investigated by high resolution transmission electron microscopy (TEM, FEI Tecnai G2 F30) and selected area diffraction (SAED) (Figure 2). Lattice fringes were clearly observed on both monoclinic and orthorhombic nanowires. The lattice fringe spacings and the diffraction patterns indicate that monoclinic nanowires can grow along [100], [010], or [001] directions (Fig-
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Figure 2. (a) A high resolution transmission electron microscope (HR-TEM) image (open arrow indicates the growth direction & inset shows image of the entire nanowire. Inset scale bar = 500 nm) and (b) a corresponding selective area electron diffraction (SAED) pattern of as-grown monoclinic nanowires. (c) HR-TEM image and (d) SAED pattern of nanowires heat-treated at 450 oC. ure 2a, b, and Figure S4, SI). Orthorhombic nanowires with growth directions of both [100] and [011] were observed (Figure 2c, d). Amorphous coatings with a thickness of a few nanometers were found along the nanowires under TEM (Figure 2a). KNbO3 nanowires were most successfully synthesized at the reaction temperature of 150 oC. The reaction temperature had a significant influence on the yield and morphology of the final KNbO3 product as shown in Figure S5, SI. Although KNbO3 nanowires were obtained at 130 oC, the yield of the product was very low. On the other hand, both the yield of the product and the portion of nanowires among the product were high at 150 oC. Some nanowires were observed from the product synthesized at 170 oC, but the portion of the nanowires was very low. Only submicron scale particulates were observed from the product of the reaction carried out at 190 oC. Considering that the growth temperature of Nb2O5-based hydrothermal reaction is 200 oC,16 the lower optimum reaction temperature (150 oC) of current Nb-based hydrothermal reaction implies that Nb has higher reactivity than Nb2O5 in the hydrothermal synthesis of KNbO3 nanowires. Moreover, we demonstrate that KNbO3 nanowires can be vertically grown on SrTiO3 substrates. A 0.5 wt% Nb-doped (100) SrTiO3 substrate (sheet resistance ~0.2 Ω/sq) with an area of 0.5 cm × 0.5 cm was used. The experiment was carried out under the same condition as described above except that the substrate was suspended using a home-made Telfon structure at approximately 1 cm height the bottom of the autoclave. (100) SrTiO3 was chosen because a lattice parameter of the cubic SrTiO3 crystal (a = 3.905 Å, JCPDS 790176) is close to lattice parameters of KNbO3. This close match of the lattice parameters is responsible for the successful growth and vertical alignment of KNbO3 nanowires. Figure 3a and b are low- and high-magnification SEM images of the KNbO3 nanowires that are vertically aligned to the (100) SrTiO3 substrates. When the nanowires were relatively long, aggregation of the tips of the nanowires were observed due to solvent evaporation (Figure 3a). The tips of the nanowires were flat and squareshaped as shown in Figure 3b. Further SEM investigation shows
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Figure 3. (a) A low and (b) a high magnification SEM image of KNbO3 nanowires grown on Nb-doped SrTiO3 substrates. (c,d) Output voltages of nanogenerators based on monoclinic and orthorhombic nanowires, respectively. (e,f) Output currents of nanogenerators based on monoclinic and orthorhombic nanowires, respectively. that a thin KNbO3 film was formed on the SrTiO3 surface, and that the film was split into sub-micrometer scale square islands and finally into nanowires (Figure S6 and S7, SI). Moreover, TEM investigation on the interface between KNbO3 and SrTiO3 showed that the growth was epitaxial (Figure S8, SI). The lattice spacing at the interface was 3.912 Å. This value gradually decreased to 3.904 Å over approximately 13 layers down towards the SrTiO3 side and gradually increased to 4.027 Å over approximately 24 layers up towards the KNbO3 side. The result indicates that the strain was localized near the KNbO3/ SrTiO3 interface (approximately ~15 nm thick). Nanogenerators fabricated with vertically aligned monoclinic nanowires showed superior performance compared to those with orthorhombic nanowires (Figure 3c–f). More specifically, the powers of monoclinic- and orthorhombic-nanowire based nanogenerators were 178.0 and 36.3 nW/cm2, respectively. Moreover, monoclinic nanowires showed both higher voltage and current generations. The power was calculated by multiplying integral areas of maximum voltage and current output of the devices. Nb doped SrTiO3 substrates and Kapton films coated with titanium (~5 nm) and platinum (~100 nm) layers were used as a bottom and a top electrode of the vertically-aligned-nanowire-based nanogenerators, respectively. The generators were tapped by applying a vertical pushing force (1 kgf; 1kgf = 9.807 N) using a pushing tester (Z-Tec. Co. Ltd, ZPS-100), of which the pushing tip has 1mm diameter.22 Voltage and current of the nanogenerators were measured using an oscilloscope (Tektronix DPO 3052) and a picoamperometer (Keithley 6485), respectively. To confirm that the signal was indeed from the nanogenerators, a switchingpolarity test was performed by swapping the polarity of electrical connections to the nanogenerators (Figure 3c).23 Our results clearly show that the monoclinic nanowires are superior to orthorhombic nanowires for nanogenerator energy harvesting applications. It
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This material is available free of charge via the Internet at http://pubs.acs.org. Details of X-ray and neutron diffraction and Rietveld refinement and additional TEM and SEM images and SHG spectra of KNbO3 nanowires are presented as supporting information. Crystallographic information file (CIF) is available for the monoclinic KNbO3.
AUTHOR INFORMATION Corresponding Author
[email protected];
[email protected] ACKNOWLEDGMENT Figure 4. (a) A schematic of an experimental setup for second harmonic generation of KNbO3 nanowires. (b) Emission spectrum of the KNbO3 nanowires (illuminated with Nd:YAG laser, λ = 1064 nm). Photographs of KNbO3 nanowires placed on a glass slide taken when the 1064 nm laser is (c) off and (d) on. has been reported that the monoclinic phase is related to nonconstrained polarization vector and high piezoelectric responses.14 Further investigation is necessary to quantitatively evaluate piezoelectric properties of nanowires with different phases. The improvement of energy conversion efficiency of nanogenerators is also expected from further optimization of the devices. Additionally, we observed nonlinear optical properties of the KNbO3 nanowires by second-harmonic generation (Figure 4). For this measurement, a thin layer of KNbO3 nanowire powder was placed between a slide glass and a cover glass. The layer thickness was approximately a few hundred microns. The sample was illuminated with a monochromatic radiation (λ = 1064 nm) from commercial Nd:YAG laser (Continuum, Powerlite Precision II 8000) and the emission spectrum from the sample was collected by a spectrometer as illustrated in Figure 4a. In the emission spectrum, the second harmonic of 1064 nm laser radiation clearly peaked at 532 nm indicating the frequency doubling capability of the KNbO3 nanowires (Figure 4b). Interestingly, we observed that the second harmonic generation (SHG) efficiency of monoclinic nanowires was ~3 times higher than that of orthorhombic nanowires (Figure S9 and S10, SI). Figure 4c and 4d show the photographs of the KNbO3 nanowire sample that were taken with the 1064 nm laser radiation off and on, respectively. Bright green light (532 nm) was generated from the sample upon the illumination by 1064 nm laser radiation. This result indicates that KNbO3 nanowires produced from our current method have decent quality and show expected nonlinear optical properties.5 In summary, high quality monoclinic KNbO3 nanowires were synthesized via a hydrothermal method using Nb precursors. Monoclinic nanowires were stable at room temperature and changed into orthorhombic phase after heat treatment at 450 oC. Moreover, vertically aligned nanowires were successfully synthesized on SrTiO3 substrates and integrated into nanogenerators, where monoclinic nanowires showed superior energy conversion characteristics to orthorhombic nanowires. In addition, nonlinear optical properties of the nanowires were observed via SHG, where monoclinic nanowires showed SHG efficiency that is ~3 times higher than that of orthorhombic nanowires. Facile and robust synthesis of the monoclinic KNbO3 nanowires demonstrated in this work may stimulate research on the alkaline-niobate nanowires for a variety of applications including energy harvesting and nanobiotechnology.
We acknowledge Prof. Brahim Dkhil for providing the crystallographic information of KNbO3 obtained at low temperature. This research was supported by the Fusion Research Program for Green Technologies through the National Research Foundation of Korea (No. 2012-0006183) and the Cooperative Research Project (B551179-12-02-00). S.-W.K. acknowledges financial support by Basic Science Research Program through the NRF, MEST (20100015035).
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