Intermediate temperature surface proton conduction on dense YSZ thin films

In this work, the electrical conductivities of sputtered 6 nm and 50 nm thick, dense yttria stabilized zirconia (YSZ) thin films are measured using impedance spectroscopy in both dry and humidified air at the intermediate temperatures of 300–650 C. While the 50 nm thick films exhibited the same conductivity in both dry and humid air, the conductivity of 6 nm thick films in humidified air was around twice as much as that in dry air at temperatures below 500 C. The conductivity increase for the 6 nm thick film was attributed to surface proton conduction, with a net conductance similar in magnitude to the bulk oxygen ion conductance. Contrary to some literature suggestions, proton conduction is believed to occur along the free surface and not through grain boundaries, since the lateral grain sizes of the 6 nm and 50 nm thick films are comparable. Interestingly, the proton conduction had an activation energy of 1.03 eV, similar to oxygen ion conduction in YSZ.


Introduction
Doped zirconia and ceria are solid electrolyte materials, normally thought to support high oxygen ion conductivity but poor conductivity for all other charged species. They are crucial components in solid oxide fuel cells, chemical sensors, and other devices. [1][2][3][4] Increased ionic conductivity at intermediate temperatures (300-650 C) is being actively pursued, especially so that the operating temperature of the devices can be reduced. 1 Besides oxygen ion and electron/hole conduction, proton conduction has also been reported in these solid electrolytes. Wagner 5 suggested that polycrystalline yttria stabilized zirconia (YSZ) is a poor proton conductor based on measurements of the permeation of H 2 . However, Nigara 6 reported that YSZ single crystals would be electron-proton mixed conductors in a H 2 -H 2 O atmosphere above 1000 C based on similar measurements.
Reports on nanocrystalline doped zirconia and ceria support the suggestion of signicant proton transport, but leave open the question of whether the conduction occurs along free surfaces, or grain boundaries, or both. Avila-Paredes 7-10 prepared nanocrystalline YSZ and gadolinia doped ceria (GDC) thin lms and found that their total conductivities were greater in wet air relative to dry air at temperatures below 150 C. The relative increase in conductivity was greater with decreasing grain size, so the authors suggested that the proton conduction occurred along grain boundaries. Shirpour 11 reported that both undoped and doped nanocrystalline ceria exhibited an increased conductivity in wet air below 200-250 C and suggested that residual porosity contributed to the protonic conductivity. Along the same lines, surface proton conduction on dense, nanocrystalline YSZ in wet air below 50 C was reported by Tande, 12 who proved proton conduction was limited to the surface via a site hopping mechanism. Oh 13 found that columnar ceria thin lms exhibited a proton conductivity nearly 4 times higher than that of nanograined bulk ceria in the temperature range of 200-400 C and suggested that the protons transported along both exposed surfaces and parallel grain boundaries.
More recently, Scherrer et al. and Gregori et al. have reported proton conduction in YSZ 14 and GDC 15 thin lms, respectively, with a thickness of a few hundred nanometers. Both of these studies found that a porous lm exhibited proton conduction at temperatures below about 300-400 C, while a dense lm showed no signicant proton conduction. Conrming these results in part, Guo 16 found no measurable proton conduction in dense YSZ thin lms with thickness of 12 nm or 25 nm, between 550-750 C. These results may suggest that dense YSZ lms have insignicant proton conduction between 300-750 C; however, these experiments have not included measurements below 550 C on ultrathin lms, where a conduction path along the free surface becomes comparable in size to the path through the bulk of the material.
In this study, we have prepared ultrathin dense YSZ thin lms by magnetron sputtering and measured the electrochemical impedance between 300-650 C. It was found that a 6 nm thick YSZ thin lm exhibited a higher total conductivity in humid air relative to dry air below 500 C, suggesting that proton conduction contributes signicantly to the overall conduction of ultrathin solid oxide electrolyte lms in humid air at intermediate temperatures. This is the rst report of dense YSZ thin lms supporting proton conduction at these temperatures (400-500 C).

Thin lm fabrication and structural characterization
YSZ thin lms were prepared using magnetron reactive cosputtering with pure metallic Zr and Y targets. The deposition atmosphere had a composition of Ar : O 2 9 : 1. The gas ow rate was maintained at 20 sccm and the working pressure was 1.33 Pa (10 mtorr). The substrates, single crystal c-plane Al 2 O 3 (MTI Corporation), were held at 650 C during the sample deposition. More details about the sample fabrication were reported previously. 17 The thicknesses of the sputtered thin lms were measured using an optical interferometer (Veeco Wyko NT9100) and veried using transmission electron microscopy (TEM) (JEOL JEM-2010F). The compositions of the lms were analyzed using X-ray photoelectron spectroscopy (XPS) (EA125 electron spectrometer, Omicron Nano Technology, Germany) and were found to be 14 mol% yttria doped for both lms. X-ray diffraction (XRD) (Rigaku Ultima IV) using conventional q-2q diffraction geometry determined the lms' phase and texture. In addition, TEM (JEOL JEM-2010F) was used to visualize the microstructures of thin lms using an acceleration voltage of 200 kV.

Electrochemical characterization
Microscale platinum electrodes were prepared using photolithography and magnetron sputtering. The interdigitated electrodes had a nger width of 25 mm, nger spacing of 25 mm, total perimeter length of 681 mm, and thickness of 200 nm. Impedance spectra (Alpha-A, Novocontrol Technologies) were collected over the frequency range from 1 Hz to 3 MHz and between temperatures of 573 K-923 K (300-650 C). The measurement atmosphere was open lab air, owing dry air and owing humidied air with a water partial pressure of z0.03 atm. The samples were measured in a sealed tube furnace and the heating rate was 100 C h À1 . The dry air was humidied by bubbling through 25 C water. The impedance spectra were analyzed by ZView soware (Version 3.3, Scribner Associates) using equivalent circuit modeling. Before measuring the conductivity of YSZ thin lms, a bare Al 2 O 3 substrate with identical electrodes was measured as a test to ensure that lm resistances were always at least an order of magnitude less than that of the substrate.

Results and discussion
where q is the Bragg angle, b is the full width at half maximum of the peak, and l is the wavelength of X-ray, the average grain size, D, was determined to be 6 nm in the 6 nm thick lm and 28 nm in the 50 nm thick lm. It should be noted that the grain sizes calculated from this equation are those perpendicular to the substrate (i.e., in the growth direction). Thus, this value can be no larger than the thickness. Bright eld transmission electron micrographs, shown in Fig. 2(a) and (b), veried the lm thicknesses as given previously. As shown in Fig. 2(a), even at a thickness of 6 nm, the lm remains dense, continuous, and nearly homogeneous in thickness. No pinholes or other microstructural features are apparent. The 6 nm thick lms show a columnar structure and the lateral grain size is roughly around 15-25 nm, which is supported by the HRTEM image ( Fig. 2(c)). The 50 nm thick lms have a columnar structure with a lateral grain size of  between 20-30 nm, which is close to the grain size perpendicular to the substrate mentioned above. Both Fig. 2(c) and (d) show that the lms had high crystal quality with a sharp YSZ/ Al 2 O 3 interface. Where grain boundaries exist, they are characterized by very low lattice mismatch angle. Based on the XRD and electron diffraction patterns, these YSZ thin lms were previously found to be nearly epitaxial, with growth along [110](111)YSZ//[10 10](0001)Al 2 O 3 . 19 Representative impedance spectra are shown in Fig. 3. The spectra of the 50 nm thick lm exhibited one clear semicircular feature at high frequency and two less well-dened features at lower frequencies. The 6 nm thick lm also exhibited a semicircular feature at high frequency, but only one discernible feature at lower frequencies. In both cases, the high frequency semicircle represents the electrolyte resistance, as concluded by the extremely low capacitance of z 10 À11 F (which arises from a parallel stray capacitance) and geometrical arguments discussed in prior work. 20 Due to the high parallel capacitance of the substrate and surrounding media relative to that of the lms, the impedance spectroscopy is unable to distinguish the resistances from the grains and grain boundaries. As shown in Fig. 3(a), the 50 nm thick lm has almost identical impedance in both dry and humidied air at 500 C. Conversely, the 6 nm thick lm exhibited a resistance in humidied air roughly half of that in dry air ( Fig. 3(b)).
With the known electrode geometry and lm thickness, the resistance obtained from equivalent circuit tting of the impedance spectra is converted to the conductivity. Fig. 4(a) presents the conductivity of the 50 nm thick YSZ thin lm in both dry and humidied air. Over the entire measured temperature range (350-650 C), the conductivity measured in humidied air was nearly same as that in dry air, whose magnitude was also similar to that of a YSZ single crystal. The activation energies in both dry and humidied air are 1.14 eV, close to that of a YSZ single crystal (1.19 eV). Below 350 C, the impedance is out of the measurement range of the spectrometer. The similarity in both dry and humidied air strongly suggests that proton conduction in the 50 nm thick YSZ lm is negligible. No signicant electron/hole conduction is expected based on the wide electrolytic domain of YSZ. 21 As shown in Fig. 5, this YSZ thin lm was measured over a range of oxygen partial pressure from 1 atm to 10 À14 atm. The conductivity was essentially constant under the entire range of oxygen partial pressures, which proves that the conductivity measured is ionic conduction rather than electronic or mixed ionic and electronic  conduction. Pure oxide ion conductivity in these lms is strongly supported by similarity of the measured conductivity to that of a YSZ single crystal.
As shown in Fig. 4(b), the conductivities of the 6 nm thick lms are nearly the same in dry and humidied air at temperatures above 550 C. The conductivity of the 6 nm lm is slightly lower than that of 50 nm thick ($60% of the conductivity of 50 nm lm). It is not clear why this is so, but may be due to differences in lateral grain size (i.e., grain boundary volume) or simply experimental variability. The activation energies of total conductivity are 1.03 eV, slightly lower than that of 50 nm thick lm (1.14 eV). Below 500 C, the 6 nm thick lm exhibited a conductivity in humidied air twice the value of that in dry air. The activation energy in humidied air is 1.09 eV, while the value in dry air is 1.13 eV. Below 400 C, the impedance of this lm falls out of the measurable range. In the temperature range of 400-500 C, the 6 nm thick lm exhibited an increased conductivity in humid air relative to that in dry air. This result suggests that the conductivity in the humidied air is a mixed oxygen ion and proton conduction; however, the conductivity in dry air is predominantly due to oxygen ion conduction over the entire measured temperature range. Under the O 2 /H 2 O atmosphere (air + 2-3 mol% H 2 O, 35-700 C), the proton conduction is expected to be dominant and the concentration of electron/ holes is negligible. 22 The conductivity increase in the presence of water is reversible. Fig. 4(b) shows two sets of measurements in dry air. These measurements were taken before and aer the measurements in open lab air, and the measurement in owing, humidied air occurred aer that. As can be seen in the plot, the conductivity reversibly changed back-and-forth between the values in owing dry air and the values in humid air.
The results given here at measurement temperatures above 550 C coincide well with the high temperature measurements given by Guo et al., 16 with nanoscale YSZ thin lms exhibiting similar conductivities in both dry and humidied air. Studies by Scherrer et al., 14 Tande et al., 12 and Avila-Paredes et al. 7,8,10 focused on proton conduction at low temperatures (from room temperature up to 400 C) and observed proton conduction especially close to room temperature. Scherrer et al. 14 reported signicant proton conduction for porous but not dense YSZ lms below 400 C. The reason for the absence of proton conduction in dense YSZ lms would be the much smaller electrode spacing. Scherrer et al. measured across-plane conductivity with the electrode spacing of only a few hundred nanometers (the lm thickness), while the electrode spacing was 25 mm in this study. So in the current direction, there is nearly no free surface, therefore no proton conductivity in previous study.
No signicant proton conduction through grain boundaries can be concluded here. The lateral grain sizes of the 6 nm thick lm and 50 nm thick lm are comparable, while the grain size perpendicular to the substrate is 6 nm for 6 nm thick lms and 28 nm for 50 nm thick lm. Thus, the volume fraction of grain boundaries is larger in the thicker lm. If proton conduction were to be occurring via the grain boundaries, then the thicker lm would exhibit greater proton conduction; this is not the behavior observed in this study.
The observation of proton conduction only in the thinner lms suggests instead that proton conduction occurs at one or both interfaces, most likely the free surface exposed to the environment. In the intermediate temperature range (400-500 C), water is likely to adsorb chemically on the oxide surfaces. 22 The associated defect chemical reaction on the surface may be This reaction produces proton defects ((OH) O c) on the oxide surface. 23,24 In addition, Raz et al. also suggested a possible model for the reaction. 22 If we assume that bulk oxygen ion conduction and surface proton conduction occur in parallel, then the resistance of surface conduction, R sur , is: where R hum and R dry are the sample resistances in humidied air and dry air, respectively. A surface conductivity is dened as: where, s sur , D, and l are the surface conductivity, the interelectrode spacing, and the total length of the electrodes, respectively. Fig. 6 shows an Arrhenius plot of the surface conductivity of the 6 nm thick lm. Below 500 C, the surface conductivity is thermally activated with an activation energy of 1.03 eV. This activation energy is close to that reported by Raz et al. 22 The difference may be explained by different proton conduction mechanism in different temperature regions. 22 Based on eqn (2) and (3), the total conductivity in humid air, s hum , is: where, s dry and t are the conductivity in dry air and the thickness of lm, respectively. As shown in Fig. 7, when t increases, s sur contributes less to the overall conductivity. This explains when the thickness of YSZ thin lms increases from 6 nm to 50 nm, the surface conduction becomes less perceptible. When measuring the sample in open laboratory air, the sample had the same conductivity as in humidied air. The similarity of these values suggests that all available adsorption sites are lled at these water vapor pressures, and that open air offers sufficient water vapor for this proton conduction mechanism at intermediate temperatures. Literature reports 25 have reported that the impedance of nanoparticle ZrO 2 lm changed four orders of magnitude when the relative humidity varied from 11% to 98%, which indicates more water is adsorbed onto the top surface of oxides when the relative humidity increases.

Conclusions
The rst observation of proton conduction on the surfaces of dense YSZ thin lms at intermediate temperatures (400-500 C) was reported here. By decreasing the thickness of YSZ thin lms to 6 nm, surface proton conduction becomes signicant. The results suggest that for ultrathin YSZ lms, not only the electron/hole conduction but also proton conduction should be considered. Proton conduction takes place along lm interfaces instead of the grain boundaries, based on the correlation of the electrical performance and microstructure of thin lms. The proton conduction had an activation energy of 1.03 eV. From this study, the thickness can be tailored to prevent or allow signicant proton conduction in dense YSZ solid electrolyte thin lms at intermediate temperature (300-650 C). Fig. 6 Arrhenius plot of the surface conductivity of the 6 nm thick YSZ thin film measured in the flowing humidified air with a water vapor partial pressure of 0.03 atm. Fig. 7 The thickness dependence of conductivity at 500 C.