PMN-PT Nanowires with a Very High Piezoelectric Constant

Apr 11, 2012 - show a piezoelectric constant (d33) up to 381 pm/V, with an average value of 373 ± 5 pm/V. This is about 15 times higher than the maxi...
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PMN-PT Nanowires with a Very High Piezoelectric Constant Shiyou Xu, Gerald Poirier, and Nan Yao* Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, United States ABSTRACT: A profound way to increase the output voltage (or power) of the piezoelectric nanogenerators is to utilize a material with higher piezoelectric constants. Here we report the synthesis of novel piezoelectric 0.72Pb(Mg1/3Nb2/3)O30.28PbTiO3 (PMN-PT) nanowires using a hydrothermal process. The unpoled single-crystal PMN-PT nanowires show a piezoelectric constant (d33) up to 381 pm/V, with an average value of 373 ± 5 pm/V. This is about 15 times higher than the maximum reported value of 1-D ZnO nanostructures and 3 times higher than the largest reported value of 1-D PZT nanostructures. These PMN-PT nanostructures are of good potential being used as the fundamental building block for higher power nanogenerators, high sensitivity nanosensors, and large strain nanoactuators. KEYWORDS: PMN-PT nanowires, hydrothermal, piezoelectric constant, PFM, nanogenerators

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anogenerators based on piezoelectric materials have attracted extensive attention because they are capable of converting mechanical, vibrational, and/or hydraulic energy into electricity for powering nanodevices and nanosystems.1 A variety of one-dimensional (1-D) piezoelectric nanostructures, such as ZnO nanowires,2 BaTiO3 nanowires,3 PZT nanofibers,4 and PVDF nanofibers,5 have been applied to the fabrication of piezoelectric nanogenerators. A nanogenerator based on ZnO nanowires, that can produce an output voltage up to 2 V, has been demonstrated to drive a small LED.6 One profound way to increase the output voltage (or power) of piezoelectric nanogenerators is to utilize piezoelectric materials with higher piezoelectric constants. One such material is new generation of single-crystal piezoelectric (1 − x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT), which exhibits a piezoelectric effect of 10 times larger (∼2500 pm/V)7 than that of conventional ceramics. Therefore, it may push the performance of these nanogenerators to a revolutionized level. It has been theoretically predicted that PMN-PT nanowires could generate higher output power with higher efficiency than ZnO nanowires; therefore, it could have better performance than ZnO nanowires as a power source, although ZnO nanowires might be a better choice as a voltage source.8 This is because PMN-PT has much larger piezoelectric coefficients and dielectric constants. Nanowires should favor higher voltage and power due to a very high ratio of length over cross area, which makes them more sensitive to small level mechanical disturbances. Another advantage to use nanowires instead of bulk materials for piezoelectric energy harvesting is the much higher strain that can be sustained by nanowires. The high flexibility and strain tolerance of nanowires could also effectively reduce the risk of potential fracture or damage of piezoelectric materials under high-frequency vibration conditions, thus broadening their safety vibration frequency and amplitude range.8,9 © 2012 American Chemical Society

To the best of our knowledge, the synthesis of 1-D PMN-PT nanostructures has not been reported yet to date. To synthesize nanostructures, a bottom-up approach is more versatile. The hydrothermal method has been used to synthesize single-crystal inorganic nanowires. This method provides a low-cost and lowtemperature way to produce high-quality single-crystal nanowires. Here, we report the synthesis of single-crystal piezoelectric PMN-PT nanowires by the hydrothermal method. The novel PMN-PT nanowires show a piezoelectric constant up to 381 pm/V, with an average value of 373 ± 5 pm/V. This is about 15 times higher than the maximum reported value of 1-D ZnO nanostructures and 3 times higher than the largest reported value of 1-D PZT nanostructures. Our study shows the good potential of these nanowires for nanogenerator, nanosensor, and nanoactuator applications. Our synthesis of PMN-PT nanowires is based on a hydrothermal method that is a versatile method to synthesize ceramic nanoparticals and nanowires.10,11 Chemical grade, stoichiometric amounts of lead acetate trihydate, magnesium 2,4-pentanedionante dehydrate, niobium ethoxide, titanium diisopropoxide bisacetyl acetonate, and 1,1,1-tri(hydroxymethyl)ethane were mixed with poly(ethylene glycol)-200 and methanol mixture under stirring condition. The concentration of the resulting sol−gel was 0.01 M. 5 mL of PMN-PT sol−gel was dispersed into distilled (DI) water to form a 60 mL solution under strong stirring condition. 20 g of KOH was slowly added into this yellow homogeneous solution as the mineralizer, and a white precipitate was formed. The final product was introduced into a homemade autoclave (80 mL in volume), sealed, and kept in an oven at 200 °C for 10 h. After cooling, the yellow suspension was washed with DI water and Received: December 8, 2011 Revised: March 23, 2012 Published: April 11, 2012 2238

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Figure 1. SEM and AFM images of PMN-PT nanostructures. (a) PMN-PT nanostructures consist of nanowires with size ranges from 200 to 800 nm. (b) A single nanowire is about 5 um long. (c) AFM image of a nanowire. (d) Cross-section profile of a nanowire.

ethanol six times to remove any unwanted element and dried at ∼100 °C in an oven overnight. A white powder which consisted of PMN-PT nanowires was obtained as the final product. The morphology of those nanowires was obtained using Quanta 200 FE-ESEM equipped with an Oxford INCA XSteam system. The X-ray diffraction (XRD) pattern was obtained using a Bruker D8 Discover diffractometer operated at grazing incidence diffraction (GID) condition. The highresolution scanning transmission electron microscopy (STEM) images, selected area diffraction (SAD), and elemental mapping were taken by a FEI ChemiSTEM 80-200. The piezoelectric measurement was performed using a Veeco Nanoman atomic force microscope (AFM). Conductive AFM tip was used to apply voltage on the nanowire. The displacement vs voltage curve was obtained by sweeping the drive voltage from 0 to 10 V. Figure 1 shows the typical morphology of the obtained PMN-PT nanostructures. Scanning electron microscopy (SEM) observations in Figure 1a reveal that the material produced consists of a large quantity of wirelike nanostructures with a typical length up to 40 μm. The yield of the nanostructures is over 90%; only a few small particles and flakes can be observed. These wirelike nanostructures form a few bundles. A larger magnification SEM image shows that not all individual nanowires have a uniform size along its longitudinal direction, as shown in Figure 1b and the AFM image in Figure 1c. The cross-section profile of this nanowire is shown in Figure 1d. At position A, the width is about 393 nm and the height is about 400 nm, and at position B, the values are 260 and 290 nm, respectively. Combining results from AFM and SEM, the cross section of individual nanowire is rectangular with a width-to-height ratio about 1. According to the measurement of a number of nanowires, the size of individual nanowires ranges from 200 to 800 nm. XRD measurement, shown in Figure 2, proves that these nanostructures are pure perovskite structured PMN-PT with lattice constants of a = 4.03 Å, b = 4.00 Å, and c = 4.02 Å, consistent with standard values of thin film PMN-PT.12 A high angle angular dark field (HAADF) image taken from a

Figure 2. XRD patterns of PMN-PT nanostructures. The diffraction peaks indicate perovskite PMN-PT.

nanowire is given in Figure 3. Figure 3a is a low-magnification DF image that shows that this nanowire has different size at both ends. The size decreased, from 372 to 665 nm, along the growth direction (as marked by the white arrow). Furthermore, the whole nanowire shows a uniform white contrast, indicating that there is no obvious defect in this nanowire. The highresolution HAADF image shown in Figure 3b further reveals that the nanowire is single crystal with near perfect structure. The lattice spacing of 4.04 Å corresponds to the (100) lattice plane, indicating this nanowire grew along [100] direction. The SAD pattern (inserted) shows that the PMN-PT nanowire is structurally uniform and single crystal. Figure 4a is an EDS spectrum of PMN-PT nanowires with indexes of all elements in PMN-PT. An EDS qualitative element mapping of a nanowire is shown in Figure 4b. All the required elements of PMN-PT distributed equally. It is noticed that potassium is still detected even though the nanowires were washed multiple times. One possible explanation could be that potassium atoms diffused into the nanowire under high temperature and high pressure environment though the crystal structure remained unchanged. 2239

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Figure 4. STEM qualitative EDS mapping of a PMN-PT nanowire. (a) EDS spectrum of PMN-PT nanowires which indexes all required elements of this materials. (b) Distribution of Pb, Ti, Nb, Mg, O, and K in the nanowire, and a uniform distribution is presented.

Table 1. EDS Quantitative Analysis of PMN-PT Nanowires

Figure 3. HAADF images of PMN-PT nanowire. (a) Lowmagnification dark field image of a nanowire which shows the geometry of it and also presents the nanowire are single crystalline. (b) HAADF image of a nanowire and inserted electron diffraction pattern prove that the nanowire is a perfect single crystal and defect free.

Another possible reason is that potassium reacts with niobium and produces KNbO3. KNbO3 is also a well-known piezoelectric material with piezoelectric constants smaller than that of PMN-PT. KNbO3 nanowires have shown a d33 constant of 7.9 pm/V.13 If this is the case, the piezoelectric properties of PMN-PT nanowires with potassium residue might be lower than expected. The quantitative analysis of these elements is shown in Table 1. The ratio of PMN and PT in nanowires is about 1.82; this is lower than the designed composition of 1.86 (65/35). And also the ratio between magnesium and niobium (4.10/8.89) is lower than 1:2, too, with the loss of magnesium. Furthermore, the concentration of lead is less than that in the precursor. If the concentration of potassium, which is 2.78 at. %, is considered, the total amount lead and potassium matches the stoichiometric amount of magnesium, niobium, and titanium. Therefore, the existence of KNbO3 in PMN-PT nanowires, as mentioned above, might be favored according to this quantitative result. It is worth mentioning that EDS has its own limitation; a further analysis for the concentration of PMN-PT nanowires should be carried on because the properties of PMN-PT are highly dependent on the ratio of the solid solution of PMN and PT as well as the purity of these materials.

element

weight %

atomic %

OK Mg K KK Ti K Nb L Pb M totals

21.33 2.00 2.18 4.80 16.57 53.12 100.00

66.46 4.10 2.78 5.00 8.89 12.78

Based on experimental results mentioned above, a possible reaction mechanism for the formation of PMN-PT nanowires under hydrothermal condition is proposed. After KOH was added into the homogeneous PMN-PT solution, a white precipitate of amorphous Pb(Mg 1/3Nb 2/3)O 3−0.5x(OH) xPbTiO3−0.5x(OH)x is instantly formed. Under hydrothermal condition, the temperature and pressure are much higher than ambient ones; therefore, the solubility of the amorphous precursor is higher than that at room temperature, and the amorphous precursor particles will dissolve into ions. Assuming HPbO2− and (Mg1/3Nb2/3)(OH)y4−y-Ti(OH)y4−y (4 < y < 6) as the soluble species in alkaline conditions, the dissolution of the precursor can be described by reaction 1: [0.72Pb(Mg1/3Nb2/3)O3 − 0.5x (OH)x ‐0.28PbTiO3 − 0.5x (OH)x ](S) + (y − 1/2x − 1)H 2O → HPbO2−(aq) + [0.65(Mg1/3Nb2/3)(OH)y 4 − y ‐0.35Ti (OH)y 4 − y ](aq) + (y − 3)H+

(1)

As time goes, the concentration of ions will increase continuously up to the minimum concentration for nucleation. 2240

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were tested by PFM. Three curves were obtained from one nanowire, and two curves were acquired from each of the other two nanowires, as shown in Table 2. The statistically averaged

The nuclei will grow by reprecipitation of the dissolved ions onto the nuclei, reducing the concentration until the equilibrium solubility is reached. The crystalline nuclei grow and gradually ripen into wirelike structures because the added polymers perform as capping agents that chemisorb at the side surfaces of the nanowires and thus inhibit their radial growth during the nucleation and growth of PMN-PT nanowires.14 The reprecipitation the soluble species to form crystalline PMN-PT is

Table 2. d33 Values from Different PMN-PT Nanowires and Locationsa slope

std error

d33 (pm/V)

0.373 88 0.376 21 0.368 14 0.374 38 0.381 46 0.365 03 0.371 96

0.0018 0.0008 0.0011 0.0012 0.0012 0.0011 0.0011

373.88 376.21 368.14 374.38 381.46 365.03 371.96

data point NW1, NW1, NW1, NW2, NW2, NW3, NW3,

HPbO2−(aq) + [0.72(Mg1/3Nb2/3)(OH)y 4 − y ‐0.28Ti (OH)y 4 − y ](aq) → [0.72PMN‐0.28PT](s) + (y − 3)OH− + 2H 2O (2)

a

The piezoelectric coefficient of single PMN-PT nanowire was measured by piezoelectric force microscopy (PFM). A detailed description of this technique can be found in ref 15. The difference is that Vecco Nanoman AFM has a built in lock-in amplifier, and the voltage vs displacement curve can be obtained by the software package directly. A drop of PMN-PT nanowires ethanol suspension was put on silicon wafer with a layer of Au/Ti and dried in air. After a nanowire was located by a contact mode AFM, the point-and-shoot function was used to land the AFM tip right on the nanowire. Then a ramp-plot function, which ramps the applied voltage from 0 to 10 V, was used to obtain a distance vs voltage sweeping curve. A typical curve of piezoelectric displacement vs drive voltage from an unpoled PMN-PT nanowire is shown in Figure 5, with a 3D

point point point point point point point

1 2 3 1 2 1 2

The statistical average value from these data is 373 ± 5 pm/V.

d33 value of these seven data points is 373 ± 5 pm/V. This value is much higher than the maximum reported value of 1-D ZnO and PZT nanostructures, which are 26.7 and 130 pm/V, respectively.15,16 An intriguing question is why the d33 value of PMN-PT nanowires is not as high as that of their bulk form. First, the nanowire was not been polarized as the bulk. Second, the highest d33 value of PMN-PT bulk materials is along [111] direction. However, according to the procedure of PFM measurement described above, the d33 value of PMN-PT nanowire could only be measured along the direction that is normal to the long axis (either [010] or [001]); as a result, a smaller piezoresponse was obtained. A much higher d33 value is expected if a polarized PMN-PT nanowire could be measured along the longitudinal direction, which will be also the direction for most applications. And also, as discussed above, the impurities in PMN-PT nanowires contribute negatively to piezoelectric properties of PMN-PT nanowires. The results presented here show the successful synthesis of piezoelectric PMN-PT nanowires by a low temperature hydrothermal method. A dissolve−reprecipitation mechanism could be used to explain the reaction mechanism. The nanowires are monoclinic single crystals with a rectangular cross section. The nonpolarized piezoelectric coefficient of these nanowires is up to 381 pm/V, with an average value of 373 ± 5 pm/V. This value is 15 times higher than the maximum reported value of 1-D ZnO nanostructures and 3 times higher than the largest reported value of 1-D PZT nanostructures. These PMN-PT nanowires could be used as the fundamental building blocks for nanogenerators with higher power and output voltage. It can also be used to fabricate novel piezoelectric nanosensors, nanoactuators, and ferroelectric nonvolatile memory and transistor devices.

Figure 5. Piezoelectric displacement vs voltage curve of an unpoled PMN-PT nanowire. The equation for the fitted curve is displacement = −0.03941 + 0.381 × voltage, which means the piezoelectric coefficient d33 determined from the slope of the curve is 381 pm/V. Inserted is a 3D morphology of the nanowire under PFM measurement.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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AFM image inserted. The red line presents the best linearly fitting, which shows that the piezoelectric displacement follows nicely with the drive voltage and gives a linear correlation with a standard deviation of 7 pm. The effective piezoelectric coefficient is determined to be 381 pm/V from the slope of the linear fitting. Since the direction of the displacement under the applied drive voltage is parallel to the electric field, this coefficient obtained is d33. Three randomly selected nanowires

ACKNOWLEDGMENTS This work is supported in part by the National Science Foundation-MRSEC program through the Princeton Center for Complex Materials (DMR-0819860). We are grateful for Dr. Anna Carlsson and Dr. Dong Tang for their assistant of using FEI ChemiSTEM80-200. 2241

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