Reaction Mechanisms of Ethylenediaminetetraacetic Acid and

Jul 2, 2012 - Reaction Mechanisms of Ethylenediaminetetraacetic Acid and Diethanolamine in the Precursor Solution for Producing (K, Na)NbO3 Thin Films...
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Reaction Mechanisms of Ethylenediaminetetraacetic Acid and Diethanolamine in the Precursor Solution for Producing (K, Na)NbO3 Thin Films with Outstanding Piezoelectric Properties Phoi Chin Goh,† Kui Yao,*,† and Zhong Chen‡ †

Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602 ‡ School of Materials Science and Engineering, Nanyang Technological University (NTU), Singapore 639798 ABSTRACT: An understanding of the reaction mechanisms of ethylenediaminetetraacetic acid (EDTA) and diethanolamine (DEA) for producing solution-derived (K, Na)NbO3 (KNN) thin films with outstanding piezoelectric properties and low leakage current is developed. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR) were used to analyze the interactions among EDTA, DEA, and metal cations in both precursor solutions and amorphous films. XPS analyses showed that the oxidation states of potassium, niobium, and oxygen in KNN amorphous film were enhanced with addition of EDTA and DEA in the precursor solution, which shifted closer to the oxidation states in the perovskite phase of the resulting KNN oxide film. FTIR analyses indicated that EDTA and DEA formed dative bonds with the nonhydrated potassium and sodium acetate after pyrolysis process at 330 °C while NMR analyses showed that such interactions could have occurred in the precursor solution. NMR analyses also indicated that DEA could have chelated niobium precursor. It is proposed that EDTA and DEA acted as a “bridge” that linked the metal precursors in the solution, which is critical to suppressing the volatilization of the metal cations, for achieving the reported outstanding electrical properties of the resulting KNN films. moderate temperature between 300 and 500 °C prior to the crystallization of the KNN perovskite phase.12 This phenomenon may be different from some lead-based piezoelectric systems such as PZT where the volatilization of PbO mainly occurred at temperature above 600 °C.13,14 Therefore, the approach of just adding excess potassium and sodium compounds in the precursor solution to compensate the loss of these oxides during thermal treatment processes as commonly reported in the literature4,10,11 cannot effectively improve the stoichiometry and enhance the piezoelectric properties of KNN films. This is because the volatile loss of potassium and sodium oxides cannot be well suppressed during the pyrolysis process at the moderate temperatures even before the crystallization. Recently, we reported successfully produced KNN thin films with outstandingly large piezoelectric coefficients and low leakage current through introducing ethylenediaminetetraacetic acid (EDTA) and diethanolamine (DEA) in the precursor solutions.15,16 The expansions of the KNN thin films were tested with a high resolution laser scanning vibrometer (LSV) method under an ac electric field of 150 kV/cm at 1.5 kHz. The LSV method can detect the displacement over a large area of the piezoelectric film, and the movements of both the electrically excited region (under the top electrode) and the

1. INTRODUCTION Studies on lead-free (K, Na)NbO3 (KNN)-based piezoelectric materials have become intensive in recent years due to their high Curie temperature (∼420 °C) and comparable piezoelectric coefficient1 with the commercially used lead zirconate titanate, Pb(Zr, Ti)O3 (PZT) piezoelectric materials. Their nonlead green composition makes them a potential replacement material for the lead-based piezoelectric ceramics that are currently dominating the commercial market. The development of these KNN-based materials has now been extended to thin film form for the intention to integrate them into sensor and actuator devices and microsystems. Unfortunately, the piezoelectric properties as reported for KNN thin films are typically far below those of the bulk KNN counterparts. As reported in the literature, the longitudinal piezoelectric coefficient d33 values for KNN thin films are usually in the range of 40−61 pm/V,2−6 which are often smaller than half of those of the bulk KNN ceramics (80−120 pm/ V7−9). One of the possible causes for obtaining such low piezoelectric coefficient in the KNN thin films could be related to their compositional nonstoichiometry and high leakage properties, which are caused by the volatilization of potassium and sodium during the thermal treatment, due to the high vapor pressure of the alkali compositions in the KNN thin films.4,10,11 In our previous study with thermogravimetric−mass spectroscopy (TG−MS) analysis, we found that the most aggressive volatile loss of the potassium and sodium oxides occurred at © 2012 American Chemical Society

Received: February 2, 2012 Revised: June 30, 2012 Published: July 2, 2012 15550

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substrate are simultaneously observed.17 The piezoelectric strain coefficient d33 value was then obtained by dividing the reliably determined expansion of the film with the applied voltage, while the piezoelectric voltage coefficient g33 value was obtained by dividing the d33 values with the dielectric constant of the film. The d33 values of 74.0 and 83.1 pm/V were obtained in the KNN and Li-doped KNN films, respectively, which are substantially higher than typical KNN-based piezoelectric thin films and even comparable to the KNN bulk ceramics7,8 and many lead-based thin films18,19 in the literature. Furthermore, the g33 values of 28.3 and 36.2 mmV/N as obtained in our KNN and Li-doped KNN films, respectively, are even higher than the KNN bulk ceramic1 and PZT thin film.20 TG−MS analyses showed that the introduction of EDTA and DEA in KNN precursor solution had effectively suppressed the volatility of potassium and sodium oxides at moderate temperature, which resulted in KNN films with outstanding piezoelectric performance.15 These promising results greatly draw our interest to explore and understand the role of EDTA and DEA and their interaction mechanisms with the KNN metal precursors. In this paper, we present our investigations on the reaction mechanisms of EDTA and DEA with potassium, sodium, and niobium precursors in both precursor solutions and amorphous films through X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance spectroscopy (NMR) analyses. The roles and mechanisms of EDTA and DEA for suppressing the volatilization of the metal oxides during thermal treatment and enhancing the electrical properties of the resulting KNN films will be discussed.

was diluted with 2-MOE to obtain a concentration of 0.3 M. For comparison purposes, KNN precursor solution without EDTA and DEA was also prepared (KNN-W). Ten mole percent of excess KOAc and NaOAc was added in both solutions for the compensation of the loss of potassium and sodium oxides during thermal treatment. One layer of KNN-W and KNN-E-D films was then spin coated on Pt/Ti/SiO2/Si substrate and pyrolyzed at 330 °C for 5 min. The binding energies of potassium, sodium, niobium, and oxygen elements on the surface of the KNN amorphous films were characterized by XPS (VG ESCALAB 2201-XL Imaging system, East Sussex, U.K.) in argon environment. XPS profiles of the samples were obtained using Al Kα source (1486.6 eV), and C 1s peak (284.5 eV) was used as the standard reference. The chemical interactions between EDTA, DEA, and metal cations in the KNN amorphous films were investigated through the attenuated total reflection (ATR) mode of the FTIR system (Spectrum 200 Perkin-Elmer, Waltham, MA). In NMR analyses, several additional solutions were prepared, i.e., EDTA and DEA in 2-MOE (EDTA-DEA), KOAc in EDTA−DEA solution (K−EDTA−DEA), and NaOAc in EDTA−DEA solution Na−EDTA−DEA). All solutions were dried in vacuum oven at 30 °C for 1 day to remove 2-MOE solvent. This was followed by dissolving each solution in deuterated methanol (CD3OD). 1H and 13C NMR spectra were recorded on an NMR spectrometer (Bruker AV-400, Karlsruhe, Germany) at 400 MHz at room temperature. Chemical shifts of 1 H and 13C were reported in parts per million with reference to the methanol solvent (CD3OD: 1H at 3.31 ppm; 13C at 49.15 ppm21).

2. EXPERIMENTAL PROCEDURE Figure 1 gives the flowchart for the preparation of EDTA and DEA modified KNN precursor solution (KNN-E-D). Potas-

3. RESULTS 3.1. XPS Analysis. XPS technique was used to study the binding energy of potassium, sodium, niobium, and oxygen on the surface of the amorphous KNN-E-D film as compared to the amorphous KNN-W film, with reference to the EDTA and DEA modified KNN oxide film annealed at 700 °C (KNN-O) that possessed the outstanding piezoelectric properties. XPS profiles for individual elements are presented in Figure 2. It is noted that the binding energies for all four elements in KNN-ED film are lower than the same elements in KNN-W film, which indicates that these elements have higher chemical state in KNN-E-D film as compared to KNN-W film. Based on the principle of XPS technique, it is known that going from the neutral atom to the positively charged cation usually gives rise to an increment in the value of the binding energy, while going from the neutral atom to the negatively charged anion leads to an opposite change. However, as reported by Szot et al.22 and in the NIST database, the binding energies for alkali metal such as potassium and sodium are higher for the metallic state than for the oxidized state, unlike other elements in the periodic table. Niobium was reported to have lower binding energy when bonded to more oxygens such as in the perovskite structure23,24 (niobium surrounded by six oxygens in the octahedral cage) as compared to Nb2O525 (combination of niobium surrounded by six oxygens in the octahedral and four oxygens in the tetrahedral cage). Hence, it is reasonable to observe the shift of all the four XPS peaks toward the lower binding energies. The higher chemical state of K, Na, and Nb in the amorphous KNN-E-D film could be due to the existence of oxygen rich EDTA and DEA molecules. Furthermore, the binding energies for potassium, niobium, and oxygen are closer

Figure 1. Flowchart for the preparation of EDTA and DEA modified KNN precursor solution.

sium acetate (KOAc) and sodium acetate (NaOAc) were first dissolved separately in the solvent of 2-methoxyethanol (2MOE). Subsequently, these two solutions were mixed together, followed by the addition of niobium ethoxide (NbOEt). EDTA and DEA were dissolved together in 2-MOE and then added into the KNN precursor solution. The mole ratio of EDTA, DEA, and KNN was fixed at 0.25:2.1:1.0. Finally, the solution 15551

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Figure 2. XPS profiles for (a) potassium, (b) sodium, (c) niobium, and (d) oxygen in KNN-W and KNN-E-D amorphous films pyrolyzed at 330 °C. The profiles for EDTA and DEA modified KNN oxide film (KNN-O) annealed at 700 °C are also provided for comparison.

to the binding energies of these elements in the KNN-O film (Figure 2a, c, d), even though the film is only pyrolyzed at 330 °C. This observation shows that addition of EDTA and DEA in the KNN precursor solution enhances the oxidation state of both potassium and niobium metal cations, which could promote early crystallization of the perovskite phase. Based on our previous finding,12 potassium and sodium become less volatile once the perovskite phase is formed. Hence, the volatility of potassium and sodium at moderate temperature in KNN-E-D reduced significantly as compared to KNN-W as observed in the TG−MS analysis in our previous work.15 Referring to Figure 2c, binding energies for Nb 3d5/2 and Nb 3d3/2 at 205.7 and 208.4 eV, respectively, in KNN-E-D film are about 0.6 eV lower than the respective binding energies in KNN-W film. However, these values are almost the same as the values observed in the KNN-O film for both binding energies. According to the literature, niobium has lower binding energy in the perovskite structure23,24 as compared to Nb2O525 due to more oxygens bonded to the niobium ions in the perovskite structure. This implies that the niobium ions in KNN-E-D amorphous film are surrounded by more oxygen ions than the niobium ions in KNN-W amorphous film at pyrolysis temperature of 330 °C, which makes the chemical state much closer to the perovskite phase in KNN-O film. Since both EDTA and DEA are well-known chelating agents for inorganic compounds, it is possible that EDTA and/or DEA chelate the niobium ethoxide in the KNN precursor solution, forming chelation complex compound.26 As such, the oxidation state of niobium increases and contributes to the lowering of the binding energy for niobium−oxygen bond as observed.

Chelation effect of EDTA and DEA claimed with niobium as mentioned above is usually not applicable to metal cations of Group 1A such as potassium and sodium in this KNN system. This is because alkali metal cations do not have an extra orbital for accepting the lone pair electrons from either the oxygen or nitrogen in the functional groups of the chelating agents. This point has already been understood and claimed in the literature.27−29 However, the oxidation states of potassium and sodium in this work are enhanced when EDTA and DEA are added into the KNN precursor solution whereby the binding energies for potassium and sodium in the KNN-E-D film are found shifted to a lower value as compared to KNN-W film, as shown in Figure 2a, b. Furthermore, the volatilization of potassium and sodium oxides is also effectively reduced in KNN-E-D as compared to KNN-W from the TG−MS analysis of our previous work.15 Hence, it is believed that EDTA and DEA have some chemical interactions with potassium and sodium cations in the KNN precursor solution that contributed to stabilizing them and minimizing their volatilization during the pyrolysis processes, but without forming chelation complex compound as usually happens to other types of metal cations with high valence states. 3.2. FTIR Analysis. FTIR technique was used here to further understand the chemical interactions between EDTA, DEA, and potassium/sodium acetate precursor by examining the changes of vibrational energy of the carboxylate group in KNN-E-D amorphous film in comparison to the KNN-W amorphous film. Figure 3 presents the selective range of the FTIR spectra for the respective films after pyrolysis at 330 °C. Wavenumbers of the corresponding peaks are labeled in the figure. 15552

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Figure 4. Proposed simplified drawing of dative bond (green arrow) between sodium acetate and DEA.

EDTA compounds, as proposed in Figure 5. In Figure 3, the wavenumber for H−O−H bending mode at 1634 cm−1 in Figure 3. FTIR spectra for −COO− stretching mode of the carboxylate and H−O−H bending mode of water in KNN-W and KNN-E-D films after pyrolysis at 330 °C.

In the wavenumber range between 1400 and 1700 cm−1, four peaks are observed in KNN-E-D film while only three peaks are seen in the KNN-W film. According to the standard infrared absorption reference for −COO − stretching mode of carboxylate group,30 peaks between 1550 and 1600 cm−1, and between 1400 and 1450 cm−1, are attributed to the asymmetry and symmetry −COO− stretching of the carboxylate functional group, respectively. By comparing the FTIR spectra for both KNN-E-D and KNN-W films, peaks at 1596 and 1438 cm−1 in KNN-E-D are the asymmetry and symmetry −COO − stretching of the acetate group of potassium and sodium precursors. The extra peak at 1392 cm−1 in KNN-E-D film could be the symmetry −COO− stretching of the carboxylate group of EDTA, which is consistent with the value reported in the literature.31 The asymmetry −COO− stretching of the carboxylate group of EDTA could be overlapped with other peaks in wavenumber between 1596 and 1634 cm−1. Thus, no distinct peak is observed. The peak in the range between 1600 and 1650 cm−1 that is found in both films could be attributed to the H−O−H bending of the water of crystallization in the hydrated acetate compounds, as claimed in the literature.32 Existence of hydrated potassium and sodium acetates in KNN precursor solution is reasonable because potassium and sodium acetates are well-known to be moisture sensitive compounds. As observed in Figure 3, both asymmetry and symmetry −COO− stretching of the acetate group in KNN-E-D have higher wavenumber as compared to KNN-W. The wavenumber of 1596 and 1438 cm−1, respectively, in KNN-E-D is about 43 and 37 cm−1 higher than the wavenumber of 1553 and 1407 cm−1, respectively, in KNN-W. This indicates that there could be an electron-withdrawing group attached to the acetate in KNN-E-D film that strengthened the CO bond.33 It is suspected that EDTA and/or DEA which contain the functional groups with lone pair electrons form dative bonds with potassium or sodium ions in the nonhydrated acetate compounds, as proposed in Figure 4. This is because formation of dative bond causes the attraction of the positively charged potassium or sodium ions toward EDTA and/or DEA, and the whole compound could act as the electron-withdrawing group to the −COO− group of the acetate. There could be another possible interaction between EDTA, DEA, and potassium/sodium acetate precursor, which could happen through the formation of hydrogen bond between the hydrogen of the water of crystallization in hydrated potassium/ sodium acetates and the oxygen and/or nitrogen in the DEA or

Figure 5. Proposed simplified drawing of hydrogen bonding (blue dotted line) between hydrated sodium acetate and DEA. Typically, there are three water molecules attached near the Na+ for hydrated sodium acetate.

KNN-E-D is 12 cm−1 lower than 1646 cm−1 in KNN-W. It is predicted that the lowering of wavenumber indicates that the weakening of the H−O−H bending strength is caused by the formation of hydrogen bond between the hydrogen of the water and the −OH group of other compound. This is because the −OH group of the water molecule is an electron acceptor.34 Hence, formation of hydrogen bond increases the electron density of the −OH group of water molecule in the hydrated potassium/sodium acetates, leading to the weakening of the H−O−H bending strength. As the EDTA and/or DEA compounds have many functional groups that are susceptible to hydrogen bonding, the above prediction is reasonable. 3.3. NMR Analysis. XPS and FTIR analyses above mainly focused on understanding the effect of EDTA and DEA in KNN amorphous films, after pyrolyzing at 330 °C. Both analyses showed that EDTA and DEA have chemical interactions with metal precursors in the KNN system. Hence, it is interesting to further investigate the mechanisms of EDTA and DEA with KNN precursors in the solution. An NMR technique is used in this study. Figure 6 presents a selected region of 1H NMR spectra corresponding to the hydroxyl group (−OH)35 of DEA for EDTA−DEA precursor without and with the addition of potassium or sodium acetate. The resonance peak at 5.32 ppm in EDTA−DEA is shifted to a lower value of 5.2 and 4.9 ppm when potassium and sodium acetate is introduced into the EDTA−DEA solution, respectively. Lowering of the chemical shift, δ, indicates the increase of shielding effect on H nucleus in the hydroxyl groups of DEA. As suggested in the FTIR analysis above, EDTA and/or DEA are suspected to form dative bonds with potassium and sodium acetate in the amorphous film. This prediction seems to be reasonable here because the positively charged H of the −OH of DEA could attract the negatively charged acetate group during dative bond formation between K+/Na+ and the oxygen of the −OH group of DEA (refer to Figure 4). As a result, the electron density of the H atom is 15553

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bond formation between EDTA and potassium and/or sodium acetate in KNN precursor solution is reasonable. Similarly to the chemical shift in 1H spectra for DEA, the shift of δ in Na− EDTA−DEA is expected to be more than in K−EDTA−DEA due to the smaller size of sodium ion as compared to potassium ion. As mentioned in the XPS analysis above, EDTA and DEA could have chelated the niobium ethoxide, forming chelation complex compound that can enhance the oxidation state of niobium in the KNN amorphous film. Hence, the reaction mechanism of EDTA and DEA with niobium ethoxide is explored by comparing the 1H NMR spectra of K−EDTA− DEA and KNN−EDTA−DEA solutions, as given in Figure 8. Figure 6. 1H NMR spectra in a region of 4.6−5.5 ppm which correspond to the hydroxyl group of DEA for EDTA−DEA precursors without and with addition of potassium or sodium acetate.

increased, leading to the increase in the shielding effect on the H nucleus. Since sodium ion has smaller ionic size than potassium ion, the shielding effect on the H nucleus of the −OH of DEA should be larger. Thus, δ decreases more in Na− EDTA−DEA than in K−EDTA−DEA, as shown in Figure 6. On the other hand, the possibility of dative bond formation between EDTA and the acetates as predicted from FTIR analysis is examined through the 13C NMR spectra from the same set of solutions mentioned above. Figure 7 shows the Figure 8. 1H NMR spectra in a region of 2.9−3.8 ppm which corresponds to the ethylene group of DEA for K−EDTA−DEA and KNN−EDTA−DEA precursor solutions.

The resonance peaks of both hydrogens in the ethylene group of DEA at 3.72 and 2.92 ppm in K-EDTA-DEA are increased to 3.76 and 2.98 ppm when niobium ethoxide is introduced into the solution (KNN−EDTA−DEA). The increase in δ indicates the decrease in the shielding effect of the H nucleus. During formation of complex chelation between niobium and DEA, the electrons of the negatively charged oxygen in the −OH group of DEA will be attracted toward the positively charged niobium ions, which draws the electron from the oxygen toward the niobium ions. This will also cause the drawing of electrons from the hydrogen in the neighboring ethylene groups. Consequently, the shielding effect on the hydrogen nucleus decreases and δ increases.

Figure 7. 13C NMR spectra in a region of 174−180 ppm which corresponds to the carbon in the carboxylic group of EDTA for EDTA−DEA precursors without and with potassium or sodium acetate addition.

4. DISCUSSION In KNN amorphous film after pyrolyzing at 330 °C, XPS analyses showed that EDTA and/or DEA could have chelated the niobium and enhanced its oxidation state due to more oxygen surrounding the niobium ions in the chelation complex compound. This observation was further confirmed through the shift of the 1H NMR for the hydrogen in the ethylene group of DEA. Moreover, FTIR analyses deduced that EDTA and/or DEA could form dative bond with the nonhydrated potassium and sodium acetate, and/or hydrogen bond with the hydrated potassium and sodium acetate. NMR results further indicated that the formation of the dative bonds is possible for both EDTA and DEA and can happen in the KNN precursor solution prior to the deposition of the KNN film. During the preparation of KNN−EDTA−DEA solution (Figure 1), EDTA was found undissolved in 2-MOE solvent.

spectra in the region between 174 and 180 ppm, which corresponds to the carbon in the carboxylic group of EDTA.36 The resonance peak for 13C in the acetate group, which is only observed in K-EDTA−DEA and Na−EDTA−DEA, falls at 181 ppm (not shown). From Figure 7, it is noticed that the resonance peak at 176 ppm in EDTA−DEA increases to 178.5 and 179.6 ppm in K−EDTA−DEA and Na−EDTA−DEA, respectively. The increase of δ indicates the decrease of the shielding effect on the carbon nucleus in the carboxylic group. This could be explained as the donation of electron lone pair of the oxygen in the −COO− group of EDTA to the positively charged K+/Na+ during dative bond formation reduces the electron density of the carbon in the −COO− group of EDTA. Hence, the shielding effect on the carbon nucleus decreases, leading to the increase in δ. This shows that prediction of dative 15554

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chemical network that is critical to suppressing the volatilization of the potassium and sodium compositions during the pyrolysis process at moderate temperature, which leads to the outstanding electrical properties of the resulting piezoelectric leadfree KNN thin films.

Cloudy solution remained even after stirring the mixture for a few hours at room temperature or at 100 °C. This may be due to the strong intermolecular bonds among EDTA molecules that could minimize the interaction between EDTA and 2MOE, as EDTA has many functional groups that are susceptible to hydrogen bonding. In contrast, a clear solution was obtained after DEA was introduced into EDTA and 2MOE mixture. It is thought that the amine functional group of DEA deprotonated the carboxylic group of EDTA through Lewis acid-based reaction. In the 1H NMR spectrum of EDTA−DEA solution (not shown), no resonance peak was observed in the region between 10 and 12 ppm, which corresponds to the δ of the carbon in the −COOH group of the carboxylic acid.37 This observation further proved that deprotonation of the carboxylic group of EDTA happened in the EDTA−DEA solution. Since the EDTA−DEA solution was prepared separately from the KNN solution, it is predicted that EDTA and DEA formed complex compounds that contain more than one DEA molecule attached to the carboxylic groups of EDTA. Based on the above results, it is proposed that EDTA and DEA act as a “bridge” that links the metal precursors through dative bonding, hydrogen bonding, and complex chelation in the KNN−EDTA−DEA precursor solution. Hydrogen bonding between EDTA, DEA, and the acetates could also easily form as both EDTA and DEA contain many functional groups that are susceptible to hydrogen bonding. As a result, a huge chemical network could form in the KNN−EDTA−DEA solution. This could help to immobilize the metal precursors in the precursor solution as well as in the amorphous gel (after pyrolysis at 330 °C). Consequently, the volatilization of potassium and sodium constituents is suppressed during pyrolysis processes, and KNN thin films with composition closer to the desired chemical stoichiometry can be obtained. In contrast, without addition of EDTA and DEA, potassium and sodium could not be well incorporated into the chemical network and hence are prone to volatile loss during the pyrolysis process. In short, addition of EDTA and DEA in the KNN precursor solution is critical to effectively suppressing the volatile loss of potassium and sodium compositions and thus leading to the outstanding electrical properties of the resulting KNN oxide films.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; phone 65-68745160; fax 6568720785. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the research grant support (SERC Grant No. 0921150112) from A*STAR (Agency for Science, Technology and Research), Singapore. The authors also acknowledge the XPS and NMR facilities provided by SnFPC (SERC nano Fabrication, Processing and Characterization), Institute of Materials Research and Engineering. The authors thank Dr. Zhang Zheng and Ms. Wang Zhen Mian for their assistance in conducting XPS and NMR experiments.

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REFERENCES

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5. CONCLUSION An understanding of the reaction mechanisms of ethylenediaminetetraacetic acid (EDTA) and diethanolamine (DEA) introduced in the precursor solution for producing solution-derived (K, Na)NbO3 (KNN) films with outstanding piezoelectric properties is developed. XPS analyses showed that the introduction of EDTA and DEA enhanced the oxidation state of potassium, sodium, niobium, and oxygen in the amorphous KNN film after pyrolyzing at 330 °C. The reduced binding energies of potassium, niobium, and oxygen after pyrolysis became closer to the binding energies of the respective element in the perovskite phase of the crystallized KNN oxide film after annealing. FTIR analyses showed that EDTA and DEA formed dative bonds with the nonhydrated potassium and sodium acetates in the KNN film pyrolyzed at 330 °C. NMR analyses indicated that chelation complex with niobium ethoxide and dative bond with potassium and sodium acetate, as suggested by the XPS and FTIR analyses, may have already formed in the precursor solution prior to the film deposition. It is proposed that EDTA and DEA act as a bridge that links the metal precursors in the solution, forming a huge 15555

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp301067g | J. Phys. Chem. C 2012, 116, 15550−15556