Thickness-Induced Proton-Conductivity Transition in Amorphous

Sep 10, 2010 - Karthik Krishnan , Hiroko Iwatsuki , Mitsuo Hara , Shusaku Nagano , Yuki Nagao. Journal of Materials Chemistry A 2014 2, 6895 ...
0 downloads 0 Views 2MB Size
Download PDF
5528 Chem. Mater. 2010, 22, 5528–5536 DOI:10.1021/cm101496s

Thickness-Induced Proton-Conductivity Transition in Amorphous Zirconium Phosphate Thin Films Yoshitaka Aoki,*,† Kota Ogawa,† Hiroki Habazaki,† Toyoki Kunitake,‡ Yuanzhi Li,§ Shinji Nagata,^ and Shu Yamaguchi# †

Graduate School of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan, NanoMembrane Technologies, Inc. Wako-RIKEN IP 406, 2-3-13 minami, Wako 351-0104, Japan, § Key Laboratory of Silicate Materials Science and Engineering, Wuhan University of Technology, Ministry of Education, Wuhan 430070, People’s Republic of China, ^Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan, and #Department of Materials Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡

Received May 28, 2010. Revised Manuscript Received August 13, 2010

Amorphous zirconium phosphate thin films, a-ZrP2.5Ox, revealed unique proton conductivity transition induced by reducing thickness due to the formation of highly conductive, hydrated nanolayer. The dense films made of a metaphosphate glass phase were uniformly formed over the electrode substrate by multiple spin-coating with a mixed precursor sol, as checked by TEM and RBS. When thickness d was larger than 60 nm, the proton conductivity σ across film and the activation energy Ea were not variable with d. σ abruptly increased 200 times and Ea decreases from 0.9 to 0.7 eV when d decreased from 60 to 40 nm, and it became thickness-independent again in d < 40 nm. σ of 100 nm-thick film is increased to the similar value as that of the 40 nm thick by annealing at 400 C in H2O/air. It was concluded that the conductivity transition could be associated with the hydration of metaphosphate nanolayer. The hydrated, highconductive phase was very stable only when the thickness was less than 100 nm. Therefore, the films of more than hundreds nm thickness cannot change to the high-conducting hydrated phase throughout the film thickness. These unprecedented behaviors could not be explicable with a simple model based on the core space charge or continuum structural relaxation at hetrointerface. Introduction The development of artificial solid electrolytes that possess the high ionic conductivity at relatively low temperatures is of great challenge for materials chemists, because it is essential to increase the efficiency and lifetime of solidstate electrochemical devices such as solid-state fuel cell,1,2 all-solid-state batteries,3,4 membrane reactor,5 memristic switch,6,7 and so on. Recently, more and more evidence of the importance of nanoscale structure in relation to ionic *Corresponding author. E-mail: [email protected].

(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

Steele, B. C. H.; Heinzel, A. Nature 2001, 414, 345. Jacobson, A. J. Chem. Mater. 2010, 22, 660. Armand, M.; Tarascon, J.-M. Nature 2008, 451, 652. Birke, P.; Chu, W. F.; Weppner, W. Solid State Ionics 1997, 93, 1. Badwal, S. P. S.; Ciacchi, F. T. Adv. Mater. 2001, 13, 993. Strukov, D. B.; Snider, G. S.; Stewart, D. R.; Williams, R. S. Nature 2008, 453, 80. Waser, R.; Dittmann, R.; Staikov, G.; Szot, K. Adv. Mater. 2009, 21, 2632. Sata, N.; Eberman, K.; Maier, J. Nature 2000, 408, 976. Maier, J. Nat. Mater. 2005, 4, 805. Maier, J. Prog. Solid State Chem. 1995, 23, 171. Garcia-Baririocanal, J.; Rivera-Calzada, A.; Verela, M.; Sefrioui, Z.; Iborra, E.; Leon, C.; Pennycook, S. J.; Santamaria, J. Science 2008, 321, 676. Tuller, H. L. Solid State Ionics 2000, 131, 143–157. Maier, J. Adv. Mater. 2009, 21, 2571. Schoonman, J. Solid State Ionics 2003, 157, 319. ope, A.; Sommer, E.; Birringer, R. Solid State Ionics 2001, 139, Tsch€ 255. Gil, Y.; Umurhan, O. M.; Riess, I. Solid State Ionics 2007, 178, 1. Berkemeier, F.; Abouzari, M. R. S.; Schmitz, G. Phys. Rev. B 2007, 76, 024205–1.

pubs.acs.org/cm

conductivity has been compiled,8-20 where not only strong variations in the conductivities are achieved but also qualitative effects such as changing the type of conductivity could be observed. In more recent years, it was clarified that the conductivity of some ionic glass films is dramatically tuned by reducing thickness into mesoscopic region17-21 because of the percolation of mesoscopically sized high-conducting pathways which are locally formed inside the films because of the internal heterogeneity.17,19,20 This phenomenon might be essential for amorphous solids since their heterogeneous structure provides distributed potential for ionic diffusion. Hence, it is motivated to investigate on the ionic conductivity of various amorphous thin films with a mesoscopically sized thickness. Inorganic proton conductor based on a metal oxide attracts much attention as a potential electrolyte for nextgeneration intermediate temperature solid-state fuel cell (IT-FC) that operates in a temperature range of 150400 C.22,23 Among various candidates, the pyrophosphate (18) Berkemeier, F.; Abouzari, M. R. S.; Schmitz, G. Ionics 2009, 15, 241. (19) Vegiri, A.; Varsami, C. E. J. Chem. Phys. 2004, 120, 7689. (20) Aoki, Y.; Habazaki, H.; Kunitake, T. J. Am. Chem. Soc. 2009, 131, 14399. (21) Stauffer, D.; Aharony, A. Introduction to Percolation Theory; CRC Press: Boca Raton, FL, 1991. (22) Norby, T. Solid State Ionics 1999, 125, 1. (23) Ito, N.; Iijima, M.; Kimura, K.; Iguchi, S. J. Power Sources 2005, 152, 200.

Published on Web 09/10/2010

r 2010 American Chemical Society

Article

Chem. Mater., Vol. 22, No. 19, 2010

5529

Table 1. Notation of the Zirconium Phosphate Specimens Post-Treated in Various Conditions, and the Atmosphere Used for Conductivity Measurements of Each Specimen notation

post-treatment procedures

atmosphere of conductivity measurement

ZP ZP-H ZP-H/D ZP-H/dry

; (as-prepared) anneal ZP at 400 C for 12 h in H2O/air (pH2O = 4.2 kPa) anneal ZP-H at 400 C for 2 h in D2O/air (pD2O = 4.2 kPa) anneal ZP-H at 400 C for 12 h in dry air

dry air (σdry) H2O/air (pH2O = 4.2 kPa) (σH2O) D2O/air (pD2O = 4.2 kPa) (σD2O) dry air (σdry)

and metaphosphate, such as NH4PO3,24,25 TiP2O7,26 La(PO3)/Ca2(PO3) composite27 LaP3O9,28,29 CeP2O7,30 and SnP2O7,31-33 have been extensively studied becaue of the singular anhydrous proton conductivity and superior durability. Especially, In-doped SnP2O7 is promising because it exhibits practical proton conductivity of 1  10-1 S cm-1 and excellent thermal stability in the temperature range.32,33 Various metal oxide-phosphate systems tend to form the stable pyrophosphate and/or metaphosphate glass phase in phosphate-rich side. Therefore, it is of fundamental and technological interest to study the mesoscopic situation of proton conductivity of such pyrophosphate or metaphosphate glass film. We have previously reported that amorphous zirconium phosphate thin film with Zr/P molar ratio of ∼1/2.6 reveals the enhanced proton conductivity even in nonhumidified atmosphere because of the existence of abundant native protons on acidic phosphate framework.34 Such anhydrous proton conductivity is actually important for the IT-FC and related applications.35 In this study, the anhydrous proton conductivity of amorphous zirconium phosphate thin film down was principally investigated. It was demonstrated that the film exhibits proton conductivity transition by decreasing thickness into less than sub-100 nm range. The conductivity of the as-prepared films is abruptly increased by 2 orders of magnitude at around 50 nm with decreasing thickness. This conductivity transition is related to the formation of a conductive hydrated layer which is stable in the film thickness of less than a few tens of nanometers. The size effect reported here might be commonly involved in various metaphosphate glass systems. (24) Matsui, T.; Takeshita, S.; Iriyama, Y.; Abe, T.; Ogumi, Z. J. Electrochem. Soc. 2005, 152, A167. (25) Chem, X.; Li, X.; Jiang, S.; Xia, C.; Stimming, U. Elecrtrochim. Acta 2006, 51, 6542. (26) Nalini, V.; Haugsrud, R.; Norby, T. Solid State Ionics 2010, 181, 510. (27) Zhang, G.; Yu, R.; Vyas, S.; Stettler, J.; Reimer, J. A.; Harley, G.; De Jonghe, L. C. Solid State Ionics 2008, 178, 1811. (28) Amezawa, K.; Kitajima, Y.; Tomii, Y.; Yamamoto, N. Electrochem. Solid-State Lett. 2004, 7, A511. (29) Amezawa, K.; Uchimoto, Y.; Tomii, Y. Solid State Ionics 2006, 177, 2407. (30) Sun, X.; Wang, S.; Wang, Z.; Ye, X.; Wen, T.; Huang, F. Solid State Ionics 2008, 179, 1138. (31) Nagao, M.; Takeuchi, A.; Heo, P.; Hibino, T.; Sano, M.; Tomita, A. Electrochem. Solid-State Lett. 2006, 9, A105. (32) Nagao, M.; Kamiya, T.; Heo, P.; Tomita, A.; Hibino, T.; Sano, M. J. Electrochem. Soc. 2006, 153, A1604. (33) Tomita, A.; Kajiyama, N.; Kamiya, T.; Nagano, M.; Hibino, T. J. Electrochem. Soc. 2007, 154, B1265. (34) Li, Y; Kunitake, T.; Aoki, Y.; Muto, E. Adv. Mater. 2008, 20, 2398. (35) Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Nature 2001, 410, 910.

Experimental Section Sample. Amorphous zirconium phosphate thin films were prepared from mixed precursor solutions of zirconium tetra-nbutoxide (Zr(OBu)4) (Kanto Chemical) and phosphorus pentoxide (P2O5) (Kanto Chemical) with the Zr/P atomic ratio of 1/3, as reported in elsewhere.34 The films were deposited on an electrode substrate in the layer-by-layer fashion by multiple spin-coating and hydrolysis. The details of this technique were previously reported.20,36 The precursor sol was prepared as follows. 0.2 M Zr(OBu)4 solution was prepared by dissolving Zr(OBu)4 into ethanol (EtOH), and 0.3 M P2O5 solution was prepared by dissolving P2O5 into 2-metoxyethanol (2MeOEtOH). Appropriate doses of Zr(OBu)4 solution and P2O5 solution were mixed into EtOH, and thus the Zr-P mixed precursor solution with the Zr/P molar ratio of 1/3 and the concentration of total metal atoms (Zr þ P) of 50 mM was obtained. The electrode substrate used was ITO-coated glass slide (ITO layer: 30 m thick), which was purchased from Aldrich and cleaned by sonicating in acetone before film deposition. The precursor sols were spin-coated onto this substrate at 3000 rpm for 40 s by a Mikasa 1H-D7 spin coater. The deposited gel layer was hydrolyzed by blowing hot air for 30 s (Iuchi hot gun), and the substrate was cooled to room temperature by blowing cold air for 20 s. These cycles of spin-coating, hydrolysis, and cooling were repeated 3-20 times and the gel films thus obtained were annealed at 400 C for 15 min. The combination of deposition and annealing was repeated more than 2 times, and the final annealing was performed at 450 C for 1 h. In this study, the films with thicknesses of 35-300 nm were prepared by changing the number of spin-coating cycles. The film as prepared by the aforementioned procedures was denoted as ZP (Table 1). Some of ZP specimens were post-treated by annealing at 400 C for 12 h in H2O/air that was prepared by bubbling the mixed gas at a rate of 100 cm3 min-1 through pure H2O at 30 C (pH2O = 0.042 kPa), and thus were denoted as ZP-H (Table 1). Characterization. The thickness and morphology of amorphous zirconium phosphate films were check by scanning electron microscopy (SEM) (JEOL JSM-6500F). The specimen for SEM observation was coated with Pt. The cross-section transmission electron microscopy (TEM) was performed by Hitachi HD-2000 at an acceleration voltage of 200 kV. Specimens for the TEM observation were prepared on electropolished Al plates (99.99%, 10  10  0.5 mm3) by the procedure as mentioned above, and ultrathin cross-sectional specimens were prepared by using an ultramicrotome. The composition of films was characterized by Rutherford backscattering spectrometry (RBS). The specimens for RBS measurement were also prepared on electropolished Al plates (99.99%, 10  10  0.5 mm). Temperature desorption spectroscopy (TDS) was performed for 10  10 mm2 size specimens prepared on a Si wafer using an ultrahigh vacuum chamber system (ESCO TDS1400) equipped with a quadruple mass analyzer and infrared (36) Aoki, Y.; Muto, E.; Nakao, A.; Kunitake, T. Adv. Mater. 2008, 20, 4387.

5530

Chem. Mater., Vol. 22, No. 19, 2010

lamp heater. The spectra were recorded during heating from 50 to 700 C at 30 C min-1 under the initial pressure of 2.0  10-7 Torr. The temperature of a sample was calibrated by Pt-PtRh thermocouple attached onto the sample surface. Fourier transform infrared spectroscopy (FR-IR) was carried out for the films prepared on Al plate with JASCO FT/IR-350 spectrometer in a reflectance mode. The Zr(HPO4)2 and ZrP2O7 were used as a reference for spectroscopy. Zr(HPO4)2 powder was purchased from Aldrich and ZrP2O7 was prepared by pyrolysis of Zr(HPO4)2 at 1000 C for 12 h in air.37 Conductivity. Proton conductivity of the films was measured by AC impedance method. Pt electrodes (100 nm thickness, 1 mmφ) were deposited on the top of the metal oxide films by ion etcher (Hitachi E1030) through a shadow mask to form a Pt/film/ITO stack. The electrical lead of Au fine wire (0.05 mmφ) was attached to the top and bottom electrodes by using Au paste (Nilaco). Impedance spectroscopy measurement was carried out for the stack by the frequency response analyzer (Solartron 1260) in a frequency range of 10 to 107 Hz at AC amplitude of 20 mV. The specimens were first heated to 400 C and kept at this temperature for several hours under controlled atmosphere, and then cooled to given temperatures of impedance measurement. The impedance data were recorded after an hour of thermal equilibration at each temperature. The proton conductivity σ of ZP films were measured in nonhumidified, “dry” air that was prepared by flowing a mixed gas of ultrapure O2 and Ar (99.999%) at a molar ratio of O2/Ar = 1/4 through H2O remover column at a rate of 100 cm3 min-1. The σ measured in such dry atmosphere was denoted as σdry (Table 1). The conductivity of ZP-H films were measured in H2O/air atmosphere (pH2O = 0.042 kPa), denoted by σH2O. To check the H/D isotope effect of σ, we treated ZP-H specimen at 400 C in the D2O/air prepared by bubbling the mixed gas at a rate of 100 cm3 min-1 through pure D2O at 31.5 C (pD2O = 0.042 kPa) so as to obtain a ZP-H/D film. The conductivity of ZP-H/D film was measured in the D2O/air, denoted by σD2O. ZP-H specimen was treated at 400 C in dry air for 12 h so as to obtain a ZP-H/dry film and σdry of ZP-H/dry was measured. The conditions for the post-treatments and the conductivity measurement of each specimen were summarized in Table 1.

Results The ZP films prepared here were an amorphous phase, as shown in the previous report.34 All the films uniformly grow over a wide area in thickness precision of nm without pinholes, cracks and aggregates of particles, as checked by SEM. The thickness increment of these films in a single coating cycle turned out to be about 4 nm for 50 mM solution. Figure 1 shows the cross-sectional TEM image of ZP films prepared on the Al plate. The film is made of a densely packed, homogeneous oxide layer and electron diffraction only shows the typical hallo ring of the amorphous phase. Figure 2 shows the RBS of ZP films with thickness of 40, 55, 100, and 260 nm prepared on the Al plate. RBS of the ZP films is composed of clear peaks of Zr, P and O and a small peak of impurity Hf in oxide film and the broad background attributed to the Al substrate. The simulation was performed with the fixed value of thickness (37) Matsui, T.; Kazusa, N.; Kato, Y.; Iriyama, Y.; Abe, Y.; Kikuchi, K.; Ogumi, Z. J. Power Sources 2007, 171, 483.

Aoki et al.

Figure 1. Cross-sectional TEM images of a 100 nm thick zirconium phosphate film. The inset is area-selected electron diffraction pattern from the film.

determined by SEM. The simulation of RBS gives the chemical composition of film as listed in Table 2, when the charge neutrality was compensated by proton. Here, amorphous zirconium phosphate film is described by a-ZrP2.5Ox. The best fitting was obtained with the film density of 0.89  1023 atoms cm-3 for ZP samples with any thicknesses. The Zr/P ratio is invariably greater than that of the corresponding precursor sol (Zr/P = 1/3), because the P2O5 is less reactive for the hydrolytic precipitation than Zr(OBu)4 and the unreacted fraction of the former may be removed during spin coating. It is remarkable that oxygen content of the ZP films decreases with increasing thickness, and thus the same goes for hydrogen content. Clearly, the ZP films exhibit impedance spectra characteristic of ion conducting film, that is, a small semicircle in the high-frequency region and a spike in the lowfrequency region in Cole-Cole plots (Figure 3).17,38,39 The obtained impedance spectra were analyzed by the nonlinear least-squares fitting so as to determine σ. Figure 4 shows the Arrhenius plot of σdry of ZP films with thickness d of 30-300 nm. All films reveal the linear Arrhenius relationship in the measured temperature range, but the σdry and the corresponding activation energy Eadry of ZP films is apparently varied by d. The σdry at 150 C and Eadry as a function of thickness are shown in Figure 5. In case of d > 67 nm, σdry and Eadry of films are not variable with thickness. When d decreases from 67 to 55 nm, σdry of films increases 3 times. Furthermore, σdry rises sharply and increases 30 times and Eadry decreases from 0.9 to 0.7 eV as d decreases from 55 to 50 nm. The value of σdry continuously rises until d reaches to 40 nm and the conductivity become thickness-independent in the region of d < 40 nm. As a result, the value of σdry increases 200 times by reduction of d from 60 to 40 nm. (38) Guo, X. X.; Matei, I.; Lee, J.-S.; Maier, J. Appl. Phys. Lett. 2007, 91, 103102. (39) Irvine, J. T. S.; Sinclair, D. C.; West, A. R. Adv. Mater. 1990, 2, 132.

Article

Chem. Mater., Vol. 22, No. 19, 2010

5531

Table 2. Composition of Amorphous Zirconium Phosphate Thin Films Determined by RBS Analysis thickness (nm)

as-prepared

hydrated

40 55 100 260

Zr1.00Hf0.01P2.53O9.67H2.64 Zr1.00Hf0.01P2.53O8.93H1.18 Zr1.00Hf0.01P2.53O8.83H0.98 Zr1.00Hf0.01P2.53O8.83H0.98

Zr1.00Hf0.01P2.53O9.67H2.64 Zr1.00Hf0.01P2.53O9.17H1.64 Zr1.00Hf0.01P2.53O9.00H1.23 Zr1.00Hf0.01P2.53O9.67H2.64 (inner, 30 nm) Zr1.00Hf0.01P2.53O8.83H0.98 (outer, 230 nm)

Figure 3. Cole-cole plot of ZP (O) and ZP-H (þ) of thickness (a, b 100 and (c-e) 300 nm. Panel b is the expansion of panel a, and panels d and e are the expansion of panel c. Solid line is calculated with an equivalent circuit of model A and dashed line is calculated with model B.

Figure 2. RBS spectra of 40, 55, 100, and 260 nm thick ZP and ZP-H films. (a, c, e) ZP films. (b, d, f) ZP-H films. Panels a and b are the survey, panels c and ) are the expansion of the Zr peak, and panels e and f are the expansion of the O peak. Dot is the observed data and solid line is the simulation.

Thermal desorption spectroscopy (TDS) was performed for ZP films of 100 and 40 nm thickness (Figure 6). TDS is a useful technique to check the existence of water present up to elevated temperature in solids.40 Major water-related species desorbed from films were H2O (m/z = 18) and (40) Hibino, T.; Mizutani, K.; Yajima, T.; Iwahara, H. Solid State Ionics 1992, 57, 303.

OH (m/z = 17). The amount of desorbed CH4 (m/z = 16), CO (m/z = 28), and CO2 (m/z = 44) is much smaller than that of desorbed H2O and OH in the measured temperature range, suggesting that the water does not evolve by combustion of organic contaminants. Both films exhibit three peaks of the desorbed water (H2O and OH) at around 130, 200, and 420 C. The desorbed at 130 and 200 C can be assigned to the physically adsorbed and/or chemically bound waters. The films show a clear peak of water desorption at temperatures above 400 C, indicating the existence of the proton stable at elevated temperatures. These waters present up to this temperature must be related to the stable proton conductivity of ZP in dry atmosphere as observed in Figure 4, because the measurement of conductivity in Figure 4 was carried out after heating specimens at 400 C for several hours in a dry air atmosphere. There is no clear difference in the intensity of peak at around 400 C between the 40 nm and the 100 nm. This may suggest that the concentration of water at elevated temperature in the 40 nm is larger than that in the 100 nm because the volume of the former is much smaller than that of the latter.

5532

Chem. Mater., Vol. 22, No. 19, 2010

Aoki et al.

Figure 6. TDS spectra of H2O (m/z = 18) of ZP films with thickness of 100 nm (;) and 40 nm (- - -).

Figure 4. Temperature dependency of σdry of ZP film with different thicknesses in dry air.

Figure 5. Plots of σdry at 150 C (O) and Eadry (4) of ZP films and σH2O at 150 C (b) and EaH2O (2) of ZP-H films.

The conductivity of a relatively thick ZP film was drastically changed by exposing in H2O/air at 400 C for a few hours. Figure 7 shows Arrhenius plots of proton conductivity of the 40 and 100 nm thick films post-treated under various conditions (Table 1). The conductivity of 40 nm thick film is not varied by annealing in H2O/air for several hours. On the other hand, σH2O of a 100 nm thick ZP-H film is nearly 2 orders of magnitude larger than σdry of the corresponding ZP film at temperatures below ca. 200 C and EaH2O of the former is apparently smaller than that of the latter. Above 200 C, the area-specific-resistance (ASR) of ZR-H films is too small to detect precisely with our setup. σdry of 100 nm thick ZP-H/dry film is almost same as σH2O of the ZP-H, indicating that the enhanced conductivity of ZPH is not restored to the original value of ZP even though the ZP-H was annealed again at 400 C in dry air for overnight. These suggest that the relatively thick a-ZrP2.5Ox film reacts with H2O at elevated temperatures to form the hydrated, high-conductive phase and the hydrated films do not recover

Figure 7. Temperature dependency of proton conductivity of 40 and 100 nm thick a-ZrP2.5Ox films treated under various conditions. Black symbols show 100 nm-thick film and red ones 40 nm thick film. black solid and red solid circles: σdry of ZP film. black and red open circles: σH2O of ZP-H film. 4: σdry of ZP-H/dry film. þ: σD2O of ZP-H/D film.

to the original phase, but the high value of σ of a thin film is maintained regardless of hydration treatment. The σD2O of 100 nm thick ZP-H/D (Table 1) is lower than σH2O of the ZPH by a factor of 1.2-1.4, showing that the conductivity decreases by H/D isotope exchange. This result confirms that the equilibrium between film and moisture actually takes place although σ of the hydrated ZP-H is not sensitive to the humidity. This ratio of σH2O/σD2O is closer to the square root of MD/MH ≈ 1.4, suggesting that a-ZrP2.5Ox film dominantly conducted protons by thermally activated hopping process. σH2O at 150 C and EaH2O of the hydrated ZP-H films with thickness of 40, 55, and 100 nm are also plotted in Figure 5. The σH2O of the ZP-H films with 55 and 100 nm thickness comes close to σdry of the ZP films with less than 50 nm thicknes. RBS of the ZP-H films of 40, 55, 100, and 260 nm were measured in order to check the depth profile of the films after hydration (Figure 2). The composition determined by RBS was also listed in Table 2. The chemical composition of

Article

a-ZrP2.5Ox films is clearly changed by hydration except for 40 nm-thick film. The spectra of 55 and 100 nm thick ZP-H films are replicated by a monolayer model with a constant density of 0.89  1023 atoms cm-3. The simulation indicates that oxygen contents increase by a few percent in both films by hydration, as listed in Table 2. The change of oxygen content is pronounced in 260 nm thick films. The attenuation of Zr and P peaks at the lower energy side shifts to higher energy but the intensity of O peak is rather enlarged at lower energy side, showing that the oxygen is enriched in vicinity of the film/substrate interface in a hydrated thicker film. Hence, the simulation matched well to the observed spectrum, using a bilayer model, where the inner 30 nmthick layer of Zr1.00Hf0.01P2.53O9.67H2.64 is developed beneath the 230 nm thick outer layer of Zr1.00Hf0.01P2.53O8.83H0.98. The composition of the outer layer is unchanged by hydration at 400 C but the oxygen content of the inner layer increases to the same level as that of the 40 nm thick film. These results clearly indicate that water (protons and oxide ions) accumulates in the interior of film near the film/ substrate interface rather than near the surface. The structures of hydrated films appeared by RBS is in consistent with the feature of impedance spectra. ColeCole plots of impedance spectra of ZP-H films with 100 and 300 nm thickness are also shown in Figure 3. The 100 nm thick ZP-H film exhibits a greatly reduced semicircle of the film conductivity in higher frequency region and a spike in low frequency region (Figure 3b). Consequently, the relaxation time, τ, of the semicircle decreases from 1  10-4 to 1  10-6 s. On the other hand, the 300 nm thick ZP-H film exposes a second semicircle in the intermediate frequency region in addition to the first semicircle in high frequency and the spike in low frequency. The τ of the first semicircle, ∼1  10-6 s, is in range of the 100 nm thick ZP-H film, and τ of the second semicircle, ∼4  10-4 s, is corresponding to that of the 100 nm thick ZP film. These results suggest that for the 300 nm thick ZP-H outer 80 vol % of the film maintains the original ZP phase but the inner layer is hydrated by accumulation of water in the region of few tens nm width near substrate interface. FT-IR spectra of the ZP and ZP-H films with 40, 55, and 100 nm thickness in the range of 500-2000 cm-1 are given in Figure 8A. The spectra of Zr(HPO4)2 and ZrP2O7 powders were also measured as a reference. Zr(HPO4)2 shows the IR bands related to bending and stretching mode of PO4 at 580, 980, and 1070 cm-1, and ZrP2O7 exhibits bands associated with stretching of terminal PO3 in P2O7 group at 1150 cm-1 and with motion of bridging oxygen in P-O-P at 770 and 980 cm-1. These features are in agreement with the previous reports.41-44 As can be seen, the features of a-ZrP2.5Ox films are clearly different (41) Casciola, M.; Donnadio, A.; Montanari, F.; Piaggio, P.; Valentini, V. J. Solid State Chem. 2007, 180, 1198. (42) Takei, T.; Kobayashi, Y.; Hata, H.; Yonesaki, Y.; Kumada, N.; Kinomura, N.; Mallouk, T. E. J. Am. Chem. Soc. 2006, 128, 16634. (43) Seyyidoglu, S.; Ozenbas, M.; Yazici, N.; Yilmaz, A. J. Mater. Sci. 2007, 42, 6453. (44) Petruska, E. A.; Muthu, D. V. S.; Carlson, S.; Anderson, A. M. K.; Ouyang, L.; Kruger, M. B. Solid State Commun. 2010, 150, 235.

Chem. Mater., Vol. 22, No. 19, 2010

5533

Figure 8. (A) FT-IR spectra of ZP and ZP-H films with d = 40, 55, and 100 nm and of references of zirconium orthophosphate (Zr(HPO4)2) and zirconium pyrophosphate (ZrP2O7). The solid and dashed lines indicate ZP and ZP-H, respectively. (a) Zr(HPO4)2, (b) ZrP2O7, (c, d) 40 nm, (e, f) 55 nm, and (g, h) 100 nm. (B) Results for Gaussian fitting to the stretching band of terminal phosphate groups in as-prepared film with d = 40 nm.

from those of the reference materials. The films are not hygroscopic before and after hydration as revealed by lack of bending mode of H-O-H, δH2O, at 1620 cm-1. a-ZrP2.5Ox film reveals the adsorption bands at around 540, 760, 960, and 1230 cm-1. Following numerous studies on phosphate glasses45-47 and crystals,41-44 the bands observed can be assigned as follows. The band at 540 cm-1 can be assigned to asymmetric bending of P-O-P, and the bands at 760 and 960 cm-1 can be associated with symmetric and asymmetric motions of bridging oxygen in P-O-P bonds (νP-O-P), respectively.45,46 The principal band of a-ZrP2.5Ox films at around 1230 cm-1 cannot be attributed to orthophosphate and/or pyrophosphate groups.45,46 It is reported that various metaphosphate glasses exhibit strong peak at around 1220 cm-1 in vibrational spectra because of the stretching of terminal PO2 (νPO2) in metaphosphate group.45,46,48 It is also reported that deformation of P-OH (νP-OH) appears at around 1240 cm-1.34,42 Therefore, the bands at 1230 cm-1 of films is sttributed to both of νPO2 and νP-OH. (45) Ilieva, D.; Jivov, B.; Bogachev, G.; Petkov, C.; Penkov, I.; Dimitriev, Y. J. Non-Cryst. Solid 2001, 283, 195. (46) Karmakar, B.; Kundu, P.; Dwivedi, R. N. Mater. Lett. 2001, 47, 371. (47) Hudgens, J. J.; Brow, R. K.; Tallant, D. R.; Martin, S. W. J. NonCryst. Solid 1998, 223, 21. (48) Ilieva, D.; Jivov, B.; Kovacheva, D.; Tsacheva, T.; Dimitriev, Y.; Bogachev, G.; Petkov, C. J. Non-Cryst. Solid 2001, 293-295, 562.

5534

Chem. Mater., Vol. 22, No. 19, 2010

The lower wavenumber side of the terminal PO2 stretching mode of a 40 nm thick film ranged from 800 to1300 cm-1 is broadened in comparison with the thicker films (Figure 8B). This may be indicative of the existence of the stretching mode of orthophosphate (νPO4) at around 1070 cm-1 and that by terminal PO3 (νPO3) of pyrophosphate and metaphosphate chain at 1150 cm-1. To estimate roughly the relative contents of orthophosphate and metaphosphate in films, we performed peakfitting for the IR bands ranged from 780 to 1500 cm-1 by using four Gaussian functions related to νP-O-P at 970 cm-1, νPO4 at 1070 cm-1, νPO3 at 1150 cm-1 and νPO2 and νP-OH at 1230 cm-1. It is difficult to separate νPO2 and νP-OH so that the band at 1230 cm-1 is treated as a single band of νPO2&P-OH. The centers of these functions were fixed at those values. The best fitting for a 40 nm-thick film is obtained with relative peak area of 14/24/62 for νPO4/νPO3/νPO2&P-OH in both cases of ZP and ZP-H. In thicker films, the best fitting for ZP and ZP-H films is obtained with about 10/21/69 and 13/24/63 for 55 nm thickness, and with about 10/19/71 and 12/20/68 for 100 nm thickness. The relative intensity of νPO4 and νPO3 components slightly increases with decreasing thickness, indicating that the molar fraction of orthophosphate increases with decreasing thickness. In addition, it is speculated that metaphosphate groups in thicker films tend to hydrolyze to orthophosphate through hydration at 400 C. Discussions Assuming that all of the phosphate groups in a a-ZrP2.5Ox film exist in form of orthophosphate, pyrophosphate or metaphosphate (PO3), the ideal composition should be Zr1.00Hf0.01P2.53O10.12H3.55, Zr1.00Hf0.01P2.53O8.86H1.03 and Zr1.00Hf0.01P2.53O8.35, respectively. The P/O atomic ratio of 40 nm thick ZP and ZP-H films is between orthophosphate and pyrophosphate, and the ratio of thicker ZP films is similar to pyrophosphate (Table 2). IR spectra clearly show that a large part of phosphate groups inside a film is metaphosphate and a combined-water in the film is rather few. These results indicate that a certain amount of oxygen atoms of a-ZrP2.5Ox are not belonging to phosphate groups and are bound to zirconium atoms. The additional protons for the negative charge compensation must be incorporated into films, and they may be bonded to oxygen on phosphate groups and/or zirconium oxide moiety. One remarkable feature of proton conductivity in a-ZrP2.5Ox film is the transition induced by downscaling thickness below a critical thickness, dc. The relationship between σ and proton concentration in various thickness films is summarized in Figure 9. For ZP films, σdry rapidly increases by 2 orders of magnitude at around dc = 50 nm. The film with d < 50 nm keeps high conductivity and large amounts of proton without hydration at 400 C. Both of σdry and proton concentration of ZP films with d ranged from 50 to 100 nm are smaller than those of the film with d < 50 nm before hydration. However, the

Aoki et al.

Figure 9. Scheme for the relationship between proton conductivity and proton concentration as a function of thickness in a-ZrP2.5Ox film before and after annealing at 400 C in H2O/air.

proton concentrations of such films are apparently increased by 20-40% by the hydration as following the increase of σ by 2 orders of magnitude. σH2O of the hydrated films with d of 100 nm is corresponding to σ of 40 nm thick films. Hence, dc of a-ZrP2.5Ox is increased to more than 100 nm by hydration at 400 C. Ea of conductivity decreases from 0.9 to 0.7 eV after transition, indicating that the mobility of protonic carriers is changed by hydration. Many authors reported that the protonic carriers in orthophosphate and pyrophosphate compounds, such as LaPO4,49 TiP2O7,26 and SnP2O732 are given by the hydrolysis of pyrophosphate group to orthophosphate group, as shown by eq 1 ðP2 O7 Þ2PO4 •• þ H2 O f 2ðHPO4 ÞPO4 • or 3ðP2 O7 ÞP2O7  þ H2 O f 2ðPO4 ÞP2O7 == þ 2ðHP2 O7 ÞP2O7 •

ð1Þ

As checked by FT-IR, a fraction of metaphosphate in films hydrolyzes through the hydration reaction similar to eq 1. The additional protonic carriers generated by this reaction are speculated to be responsible for the high conductivity of hydrated phase. Accordingly, it is concluded that the conductivity transition of a-ZrP2.5Ox films is induced by the structural modification by hydration. In d > 100 nm, proton concentrations are enriched by hydration only in the inner layer within the range of a few tens of nanometers from substrate interface even though that of the outer layer remains unchanged. This result is surprising, because the hydration of the inner layer needs larger activation energy than that of outer layer because of the longer diffusion length. Apparently, the film having sufficiently small thickness is intrinsically hydrated regardless of annealing in H2O/air. This strongly suggests that the highly conductive hydrated phase is rather stable when its layer thickness is in range of few tens of nanometers. Recently, it has been reported that the concentration of moisture absorbed in polymer thin films on various substrates is enriched or diluted in the vicinity (49) Amezawa, K.; Maekawa, H..; Tomii, Y.; Yamamoto, N. Solid State Ionics 2001, 145, 233.

Article

Figure 10. Thickness-dependent conductivity, Σ (d), calculated by a space charge model (- - -) and a exponential relaxation model (;). σdry at 150 C of ZP films, which has already been shown in Figure 6, is also plotted (O).

of polymer/substrate interface because of lowering the excess energy at interface.50-52 If the hydration of ZP film is driven by such an effect, the high-conductive, hydrated phase must exponentially decay with increasing distance from the substrate. The conductivity at x, σ(x), can be represented as follows      x ðx - dÞ σðxÞ ¼ σh exp þ exp θ θ      x ðx - dÞ þ σ¥ 2 - exp - exp ð2Þ θ θ Here, x is distance from one electrode interface (x = 0) to the other interface (x = d), σh is the conductivity of highconductive phase, and θ is dispersion with a dimension of length. The overall conductivity Σ(d) across film can be replicated by the connection of σ(x) in series, and thus it is calculated by following equation. "Z #-1 d 1 dx ΣðdÞ ¼ d ð3Þ 0 σðxÞ By using eqs 2 and 3, Σ(d) is calculated with varied parameters of σh, σ¥, and θ in order to replicate the σdry at 150 C of ZP films. The results for simulation by this exponential relaxation model are displayed in Figure 10. An optimal fitting can be achieved with σ¥ = 7.8  10-9 S cm-1, σh = 2.110-5 S cm-1 and θ=3.3 nm. Apparently, the calculated curve does not suit to the abrupt transition behavior of measured conductivity of ZP films. In eq 2, the structural modification continuously decays with increasing x. The oxygen concentration gradient in ZP film was not obvious from the preceding results. In addition, a relatively thick a-ZrP2.5Ox layer clearly splits into the hydrated inner layer (50) Vogt, B. D.; Soles, C. L.; Jones, R. L.; Wang, C.-Y.; Lin, E. K.; Wu, W.-I.; Satija, S. K. Langmuir 2004, 20, 5285. (51) Vogt, B. D.; Soles, C. L.; Lee, H.-J.; Lin, E. K.; Wu, W.-I. Langmuir 2004, 20, 1453. (52) Vogt, B. D.; Soles, C. L.; Lee, H.-J.; Lin, E. K.; Wu, W.-I. Polymer 2005, 46, 1635.

Chem. Mater., Vol. 22, No. 19, 2010

5535

and the unhydrated outer layer by hydration reaction. These results may suggest that such a continuum structure relaxation model does not fit to the conductivity transition of a-ZrP2.5Ox film. In recent year, the remarkable enhancement of ionic transport of multilayer thin film due to forming the ionic carriers with higher mobility by the formation of spacecharge region in vicinity of heterointerfaces, so-called “nanoionics effect”, has been reported for crystalline solids by many authors.8,53-55 In a-ZrP2.5Ox, if the negatively charged phosphate groups are segregated in narrow layer at electrode interface, the redistribution of mobile protons is involved to screen the negative interface charge. As a consequence, minor protonic carriers with high mobility are brought inside this space charge region, which leads to an elevated protonic conductivity. The σ(x) due to space-charge can be written by the following equations54 σðxÞ ¼ eμH C¥H exp½ZðxÞ þ σ ¥

ð4Þ

Here, CH¥ is the concentration of proton in the center of a sufficiently thick glass, and Z(x) is the normalized electric potential given by Poisson’s equation.54 Thus, Σ(d ) can be calculated from eqs 2 and 3, assuming that the carrier mobility μH is independent of x. A reasonable description for the experimental conductivity is not achieved by using various values of λ and φ(0). The Σ (d ) calculated with λ = 30 nm, σ¥ = 1.0  10-8 S cm-1 and φ(0) = 800 mV is shown in Figure 9. Hence, the conductivity transition of our film cannot be explicable by a simple space-charge model. Conclusions In summary, it is demonstrated that amorphous ZrP2.5Ox thin films were a thermally stable, anhydrous proton conductor since these retain the abundant protons bound to metaphosphate glass framework. Furthermore, ZrP2.5Ox films reveal unique conductivity transition as followed by decrease of thickness, which is related to the water content inside film. The proton conductivity in dry air is not dependent on the thickness in d > 60 nm, but it dramatically increases as reducing thickness into below 60 nm. As a consequence, the conductivity of 40 nm thick film is 200 times higher than that of 60 nm thick film. Furthermore, the conductivity of films with d of 100 nm increases by 2 orders of magnitude by hydration treatment and becomes as high as that of the as-prepared 40 nm thick film, though the conductivity of 40 nm thick film is kept at high level regardless of hydration. The elevated conductivity of thin films can attribute to the structural modification of metaphosphate glass framework by hydration. The hydrated, high-conductive phase is thermally stable and does not restore to the original (53) Kim, S.; Maier, J. J. Electrochem. Soc. 2002, 149, J73. (54) Guo, X.; Matei, I.; Jamnik, J.; Lee, J.-S.; Maier, J. Phys Rev. B 2007, 76, 125429–1. (55) Azad, S.; Marina, O. A.; Wang, C. M.; Saraf, L.; Shutthanandan, V.; McCready, D. E.; El-Azab, A.; Jaffe, J. E.; Engelhard, M. H.; Peden, C. H. F.; Thevuthasan, S. Appl. Phys. Lett. 2005, 86, 131906.

5536

Chem. Mater., Vol. 22, No. 19, 2010

unhydrated phase by annealing at 400 C in dry air, but it is stabilized only as the thickness is less than a few tens of nanometers. Our findings strongly suggest that the hydration of metaphosphate films is encouraged in the nanometer-thick layer in proximity to the electrode rather than in the surface layer exposed directly to moistures. Consequently, the highly conductive, hydrated phase is confined within the region of a few tens of nanometers thickness from electrode interface, so that the relatively thick films reveal the phase separation between the inner hydrated nanolayer and the outer unhydrated layer. To the best of our knowledge, this is the first example of the proton-conductivity transition triggered by the size-confined hydration effect. The origin for this size-confinement is still unclear, and is not readily explained by the exponential decay of structure induced by the fixed

Aoki et al.

charge or free energy at the interface. These phenomena reported here are speculated to be operative in various phosphate-based amorphous proton conductors, since the Gibbs energy of amorphous compounds must be larger than that of the crystalline materials under same composition. The current results pose an opportunity to create the high-proton-conductivity electrolyte based on the hydrated metaphosphate glass. Acknowledgment. Part of this work was supported by the Use-of-UVSOR Facility Program (B8A1, 2009) of the Institute of Molecular Science. This work was financially supported by the Grant-in-Aid of JSPS for Scientific Research on Young Scientists (B) and by the Global COE Program (Project No. B01: Catalysis as the Basis for Innovation in Materials Science) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.