Microscopy, 2014, 463–467 doi: 10.1093/jmicro/dfu095 Advance Access Publication Date: 20 October 2014
Stability of {111}Pd/{0002}ZnO polar interface formed by internal oxidation of Pd–Zn alloys Kei Watanabe, Yuji Kunisada, and Norihito Sakaguchi*
*To whom correspondence should be addressed. E-mail: [email protected] Received 31 July 2014; Accepted 29 September 2014
Abstract We investigated the stability of Pd/ZnO polar interfaces formed by internal oxidation of Pd–Zn alloys by using high-resolution transmission electron microscopy, electron energy loss spectroscopy and convergent-beam electron diffraction. At 1273 K, a ð111ÞPd =ð0002Þ ZnO polar interface defaceted and transformed into a curved interface, while another (111)Pd/ (0002)ZnO polar interface retained its flatness. The ð111ÞPd =ð0002Þ ZnO polar interface lost some stability over non-polar interfaces at 1273 K, while the (111)Pd/(0002)ZnO polar interface remained stable. Key words: internal oxidation, polar interface, interfacial structure, convergent-beam electron diffraction
Introduction Internal oxidation is a process in which precipitates of alloying oxides form within a solvent metal matrix. It is usually unfavourable, because it degrades mechanical strength and corrosion resistance [1–3]. To inhibit internal oxides, many have investigated microstructural methods [1–6], a convenient way to control metal/oxide interfacial structures because the coherency and chemical bonding of the interfacial elements strongly influence the mechanical and chemical properties of the material. In general, metal/oxide interfaces formed by internal oxidation tend to be oxygen-terminated (O-terminated). However, some reports have indicated that the oxygen occupancy at the metal/oxide interface depends on the oxidation atmosphere [7,8]. For example, Pippel et al. reported that annealing internally oxidized Ag–Mg alloys in a vacuum decreased the oxygen occupancy at the Ag/MgO interface by 50% [8]. The species of terminating elements at the Pd/ZnO interfaces also depends on oxidization conditions [9–11]. Because the stable crystalline structure of ZnO is wurtzite,
which has no inversion symmetry, its (0002) and ð0002Þ polar surfaces are not identical, even though the same species terminates each ZnO surface. The physical properties of metal/ZnO interfaces depend much on the crystalline polarity and the terminating species. Our previous study revealed that one Pd/ZnO polar interface formed by internal oxidation of a Pd–Zn alloy was Zn-terminated, while another was O-terminated [12]. Both the Zn- and O-terminated interfaces were stable in the Pd– ZnO system at the present oxidation condition; however, the stability of both polar interfaces should change with the oxidation temperature or partial pressure of oxygen. In this study, we investigated the stability of each polar interface at higher oxidation temperatures. We used high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and convergent-beam electron diffraction (CBED) to investigate the structure of the Pd/ZnO interface and the crystalline polarity of the ZnO precipitate. Using these measurements, we discuss changes in the interfacial structures on one side of the polar interfaces.
© The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: [email protected]
463
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
464
Methods
Results and discussion Figure 1a shows a typical TEM image of the ZnO particles after internal oxidation at 1173 K for 100 h. The ZnO precipitates in samples oxidized at 1173 K and at lower
temperatures had similar shapes. We confirmed that atomically flat (111)Pd/{0002}ZnO polar interfaces (indicated by arrows) appeared at both ends of the precipitates, forming because of the low energy of the polar interfaces in the present system. As shown in the interfacial atomic model (Fig. 1c), we confirmed using HRTEM that, for the samples internally oxidized below 1173 K, the (111)Pd/(0002)ZnO interface (Zn face) is Zn-terminated and the ð111ÞPd =ð0002Þ ZnO interface (O face) is O-terminated [12]. Both polar interfaces are atomically flat below 1173 K [12]. Figure 1d shows a typical oxygen K-edge ELNES acquired at the interior and O-terminated interface of ZnO precipitates after internal oxidation of the Pd–Zn alloy [12]. This figure also includes O-K ELNES calculated from the former atomic model [13]. The pre-edge peak at 535 eV only appeared in the O-terminated interfacial O-K ELNES; the calculated ELNES spectrum indicates that the pre-edge peak reflects the hybridization between Pd-d and O-p orbitals across the interface. Figure 2a shows a TEM image of a ZnO precipitate formed after internal oxidation at 1273 K for 100 h. They appeared to be different from typical ZnO precipitates, as shown in Fig. 1a. After oxidation at 1273 K, one (111)Pd/{0002}ZnO polar interface defaceted and transformed into a curved interface, whereas the other retained its flatness. To determine the crystalline polarity of the ZnO, the CBED technique is acceptable because the patterns in the CBED disks obviously differ
Fig. 1. (a) TEM image of ZnO precipitates after internal oxidation at 1173 K for 100 h. The (111)Pd/{0002}ZnO polar interfaces are indicated by arrows. (b) HRTEM images of upper and lower polar interfaces. (c) Atomic model of the (111)Pd/{0002}ZnO polar interface; Pd, Zn and O atoms are indicated by yellow, blue and red spheres, respectively. (d) Oxygen K-edge ELNES acquired from O-terminated polar interface and bulk ZnO [12,13]. A pre-edge peak is indicated by an arrow in the interfacial spectrum.
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
First, a Pd rod (99.99%) was cold-rolled to a thickness of 0.1 mm. To obtain the Pd–Zn alloy, the rolled Pd sheet was heated at 1273 K for 100 h with a Zn wire (99.95%) in an evacuated quartz tube. The resultant alloy had a Zn concentration of 18 at.%. A 3-mm diameter disc was punched from the Pd–Zn sheet and electropolished in a solution of 80% acetic acid and 20% perchloric acid at 285 K, using a twin-jet technique. The thin-foil sample was then internally oxidized for 100 h at 1173 or 1273 K in air. The sample was etched by ion milling for several minutes to remove the surface contaminant layer. A high-voltage transmission electron microscope (JEOL JEM-ARM-1300) and a field-emission transmission electron microscope (JEOL JEM-2010F) were used to obtain HRTEM images, EELS spectra and CBED patterns. MacTEMPAS software was used to simulate the CBED patterns of the wurtzite ZnO. WIEN2k code with TELNES2 software was used to calculate the oxygen K-edge energy-loss near-edge structure (ELNES) of the ZnO.
Microscopy, 2014, Vol. 63, No. 6
Microscopy, 2014, Vol. 63, No. 6
Fig. 3. (a) TEM image of ZnO precipitate after internal oxidation at 1273 K. (b) Experimental CBED pattern of the ZnO precipitate. (c) HRTEM image of the curved interface. Arrows indicate many small steps consisting of ð111ÞPd =ð0002Þ ZnO interfaces. (d) HRTEM image of (111)Pd/(0002)ZnO polar interface.
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
Fig. 2. (a) TEM image of ZnO precipitates after internal oxidation at 1273 K for 100 h. (111)Pd/{0002}ZnO polar interfaces are indicated by arrows. (b) Experimental and (c) calculated CBED patterns of the ZnO precipitates.
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pre-edge peak in oxygen K-edge ELNES is caused by strong chemical bonding and hybridization of the Pd-d and O-p orbits, which indicates Pd–O chemical bonding, even at the curved interface, because the ð111ÞPd =ð0002Þ ZnO polar interfaces exist locally along the curved interface. At 1273 K, internal oxidation defaceted the ð111ÞPd =ð0002Þ ZnO polar interface, transforming it into a curved interface. This behaviour indicates that the interface, originally planar at 1173 K, was unstable at 1273 K. At this temperature, ZnO grew more rapidly than at 1173 K because of the Zn atoms diffused more quickly. However, almost all the Zn solute precipitated as ZnO, even below 1173 K. Larger ZnO particles grew by Ostwald ripening, whereas smaller ZnO particles dissolved into the matrix. Below 1173 K, decomposition started from the edge (nonpolar interfaces) of ZnO because both the (111)Pd/ {0002}ZnO polar interfaces remained flat. In contrast, at 1273 K, small ZnO particles decomposed at the ð111ÞPd =ð0002Þ ZnO polar interface and at other non-polar interfaces. Thus, the ð111ÞPd =ð0002Þ ZnO polar interface defaceted and transformed into a curved interface. This behaviour indicates that, at 1273 K, the energetic advantage of the polar interface was lost. (111)Pd/{0002}ZnO polar interfaces formed by internal oxidation were typically those with the lowest energy, even though the (111)Pd/(0002)ZnO
Fig. 4. (a) TEM image of ZnO precipitates after internal oxidation at 1273 K for 100 h. (b) Oxygen K-edge ELNES acquired from interfaces and bulk ZnO. The colours of the circles in (a) correspond to the colours of the spectra in (b).
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
reflections [14]. Thus, we between the (0002) and (0002) acquired the CBED patterns of the ZnO precipitates to investigate their crystalline polarity. Figure 2b shows an experimen tally obtained ½1100 CBED pattern, and Fig. 2c shows a calculated ½1100 CBED pattern of a 40-nm-thick ZnO crystal. The CBED patterns and TEM image in Fig. 2a suggest that the curved interfaces were always the ð111ÞPd =ð0002Þ ZnO polar interface (O face). As shown in Fig. 3a, we determined the polarity of another ZnO precipitate by CBED, revealing defa ceting at the ð111ÞPd =ð0002Þ ZnO polar interface, similar to former results. Figure 3c shows typical HRTEM images of both polar interfaces. The (111)Pd/(0002)ZnO interface remained flat, even at an oxidation temperature of 1273 K. However, many small steps appear along the curved interface; between these steps were short ð111ÞPd =ð0002Þ ZnO polar interfaces. To investigate the bonding nature of oxygen atoms near the {0002}ZnO polar interfaces, we collected EELS spectra. Figure 4a shows oxygen K-edge ELNES acquired from the ZnO precipitate interfaces. The red line in Fig. 4b indicates the spectrum acquired from the curved interface, while the blue line indicates that from the (111)Pd/(0002)ZnO interface. As a reference, the spectrum of bulk ZnO is shown in green. A pre-edge feature, indicated by the arrow in Fig. 4b, appears in the spectrum of the curved interface, but not in those acquired from the (111)Pd/(0002)ZnO interface or bulk ZnO. The
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ZnO
polar interface was not completely lost at 1273 K. Another explanation is that the steps formed from the faceting transition of the curved interface, during air cooling of the sample after oxidation. Excess oxygen atoms in the matrix would have begun segregating at the curved interface during cooling, and many ð111ÞPd =ð0002Þ ZnO polar interface facets would have formed there. Further experimental and theoretical insight is required to confirm these explanations.
Concluding remarks Using HRTEM, EELS and CBED, we investigated the atomic and chemical structures of Pd/ZnO polar interfaces formed by internal oxidation of Pd–Zn alloys. At oxidation temperatures
Stability of {111}Pd/{0002}ZnO polar interface formed by internal oxidation of Pd–Zn alloys Kei Watanabe, Yuji Kunisada, and Norihito Sakaguchi*
*To whom correspondence should be addressed. E-mail: [email protected] Received 31 July 2014; Accepted 29 September 2014
Abstract We investigated the stability of Pd/ZnO polar interfaces formed by internal oxidation of Pd–Zn alloys by using high-resolution transmission electron microscopy, electron energy loss spectroscopy and convergent-beam electron diffraction. At 1273 K, a ð111ÞPd =ð0002Þ ZnO polar interface defaceted and transformed into a curved interface, while another (111)Pd/ (0002)ZnO polar interface retained its flatness. The ð111ÞPd =ð0002Þ ZnO polar interface lost some stability over non-polar interfaces at 1273 K, while the (111)Pd/(0002)ZnO polar interface remained stable. Key words: internal oxidation, polar interface, interfacial structure, convergent-beam electron diffraction
Introduction Internal oxidation is a process in which precipitates of alloying oxides form within a solvent metal matrix. It is usually unfavourable, because it degrades mechanical strength and corrosion resistance [1–3]. To inhibit internal oxides, many have investigated microstructural methods [1–6], a convenient way to control metal/oxide interfacial structures because the coherency and chemical bonding of the interfacial elements strongly influence the mechanical and chemical properties of the material. In general, metal/oxide interfaces formed by internal oxidation tend to be oxygen-terminated (O-terminated). However, some reports have indicated that the oxygen occupancy at the metal/oxide interface depends on the oxidation atmosphere [7,8]. For example, Pippel et al. reported that annealing internally oxidized Ag–Mg alloys in a vacuum decreased the oxygen occupancy at the Ag/MgO interface by 50% [8]. The species of terminating elements at the Pd/ZnO interfaces also depends on oxidization conditions [9–11]. Because the stable crystalline structure of ZnO is wurtzite,
which has no inversion symmetry, its (0002) and ð0002Þ polar surfaces are not identical, even though the same species terminates each ZnO surface. The physical properties of metal/ZnO interfaces depend much on the crystalline polarity and the terminating species. Our previous study revealed that one Pd/ZnO polar interface formed by internal oxidation of a Pd–Zn alloy was Zn-terminated, while another was O-terminated [12]. Both the Zn- and O-terminated interfaces were stable in the Pd– ZnO system at the present oxidation condition; however, the stability of both polar interfaces should change with the oxidation temperature or partial pressure of oxygen. In this study, we investigated the stability of each polar interface at higher oxidation temperatures. We used high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS) and convergent-beam electron diffraction (CBED) to investigate the structure of the Pd/ZnO interface and the crystalline polarity of the ZnO precipitate. Using these measurements, we discuss changes in the interfacial structures on one side of the polar interfaces.
© The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: [email protected]
463
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
464
Methods
Results and discussion Figure 1a shows a typical TEM image of the ZnO particles after internal oxidation at 1173 K for 100 h. The ZnO precipitates in samples oxidized at 1173 K and at lower
temperatures had similar shapes. We confirmed that atomically flat (111)Pd/{0002}ZnO polar interfaces (indicated by arrows) appeared at both ends of the precipitates, forming because of the low energy of the polar interfaces in the present system. As shown in the interfacial atomic model (Fig. 1c), we confirmed using HRTEM that, for the samples internally oxidized below 1173 K, the (111)Pd/(0002)ZnO interface (Zn face) is Zn-terminated and the ð111ÞPd =ð0002Þ ZnO interface (O face) is O-terminated [12]. Both polar interfaces are atomically flat below 1173 K [12]. Figure 1d shows a typical oxygen K-edge ELNES acquired at the interior and O-terminated interface of ZnO precipitates after internal oxidation of the Pd–Zn alloy [12]. This figure also includes O-K ELNES calculated from the former atomic model [13]. The pre-edge peak at 535 eV only appeared in the O-terminated interfacial O-K ELNES; the calculated ELNES spectrum indicates that the pre-edge peak reflects the hybridization between Pd-d and O-p orbitals across the interface. Figure 2a shows a TEM image of a ZnO precipitate formed after internal oxidation at 1273 K for 100 h. They appeared to be different from typical ZnO precipitates, as shown in Fig. 1a. After oxidation at 1273 K, one (111)Pd/{0002}ZnO polar interface defaceted and transformed into a curved interface, whereas the other retained its flatness. To determine the crystalline polarity of the ZnO, the CBED technique is acceptable because the patterns in the CBED disks obviously differ
Fig. 1. (a) TEM image of ZnO precipitates after internal oxidation at 1173 K for 100 h. The (111)Pd/{0002}ZnO polar interfaces are indicated by arrows. (b) HRTEM images of upper and lower polar interfaces. (c) Atomic model of the (111)Pd/{0002}ZnO polar interface; Pd, Zn and O atoms are indicated by yellow, blue and red spheres, respectively. (d) Oxygen K-edge ELNES acquired from O-terminated polar interface and bulk ZnO [12,13]. A pre-edge peak is indicated by an arrow in the interfacial spectrum.
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
First, a Pd rod (99.99%) was cold-rolled to a thickness of 0.1 mm. To obtain the Pd–Zn alloy, the rolled Pd sheet was heated at 1273 K for 100 h with a Zn wire (99.95%) in an evacuated quartz tube. The resultant alloy had a Zn concentration of 18 at.%. A 3-mm diameter disc was punched from the Pd–Zn sheet and electropolished in a solution of 80% acetic acid and 20% perchloric acid at 285 K, using a twin-jet technique. The thin-foil sample was then internally oxidized for 100 h at 1173 or 1273 K in air. The sample was etched by ion milling for several minutes to remove the surface contaminant layer. A high-voltage transmission electron microscope (JEOL JEM-ARM-1300) and a field-emission transmission electron microscope (JEOL JEM-2010F) were used to obtain HRTEM images, EELS spectra and CBED patterns. MacTEMPAS software was used to simulate the CBED patterns of the wurtzite ZnO. WIEN2k code with TELNES2 software was used to calculate the oxygen K-edge energy-loss near-edge structure (ELNES) of the ZnO.
Microscopy, 2014, Vol. 63, No. 6
Microscopy, 2014, Vol. 63, No. 6
Fig. 3. (a) TEM image of ZnO precipitate after internal oxidation at 1273 K. (b) Experimental CBED pattern of the ZnO precipitate. (c) HRTEM image of the curved interface. Arrows indicate many small steps consisting of ð111ÞPd =ð0002Þ ZnO interfaces. (d) HRTEM image of (111)Pd/(0002)ZnO polar interface.
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
Fig. 2. (a) TEM image of ZnO precipitates after internal oxidation at 1273 K for 100 h. (111)Pd/{0002}ZnO polar interfaces are indicated by arrows. (b) Experimental and (c) calculated CBED patterns of the ZnO precipitates.
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pre-edge peak in oxygen K-edge ELNES is caused by strong chemical bonding and hybridization of the Pd-d and O-p orbits, which indicates Pd–O chemical bonding, even at the curved interface, because the ð111ÞPd =ð0002Þ ZnO polar interfaces exist locally along the curved interface. At 1273 K, internal oxidation defaceted the ð111ÞPd =ð0002Þ ZnO polar interface, transforming it into a curved interface. This behaviour indicates that the interface, originally planar at 1173 K, was unstable at 1273 K. At this temperature, ZnO grew more rapidly than at 1173 K because of the Zn atoms diffused more quickly. However, almost all the Zn solute precipitated as ZnO, even below 1173 K. Larger ZnO particles grew by Ostwald ripening, whereas smaller ZnO particles dissolved into the matrix. Below 1173 K, decomposition started from the edge (nonpolar interfaces) of ZnO because both the (111)Pd/ {0002}ZnO polar interfaces remained flat. In contrast, at 1273 K, small ZnO particles decomposed at the ð111ÞPd =ð0002Þ ZnO polar interface and at other non-polar interfaces. Thus, the ð111ÞPd =ð0002Þ ZnO polar interface defaceted and transformed into a curved interface. This behaviour indicates that, at 1273 K, the energetic advantage of the polar interface was lost. (111)Pd/{0002}ZnO polar interfaces formed by internal oxidation were typically those with the lowest energy, even though the (111)Pd/(0002)ZnO
Fig. 4. (a) TEM image of ZnO precipitates after internal oxidation at 1273 K for 100 h. (b) Oxygen K-edge ELNES acquired from interfaces and bulk ZnO. The colours of the circles in (a) correspond to the colours of the spectra in (b).
Downloaded from http://jmicro.oxfordjournals.org/ at The Australian National University on January 20, 2015
reflections [14]. Thus, we between the (0002) and (0002) acquired the CBED patterns of the ZnO precipitates to investigate their crystalline polarity. Figure 2b shows an experimen tally obtained ½1100 CBED pattern, and Fig. 2c shows a calculated ½1100 CBED pattern of a 40-nm-thick ZnO crystal. The CBED patterns and TEM image in Fig. 2a suggest that the curved interfaces were always the ð111ÞPd =ð0002Þ ZnO polar interface (O face). As shown in Fig. 3a, we determined the polarity of another ZnO precipitate by CBED, revealing defa ceting at the ð111ÞPd =ð0002Þ ZnO polar interface, similar to former results. Figure 3c shows typical HRTEM images of both polar interfaces. The (111)Pd/(0002)ZnO interface remained flat, even at an oxidation temperature of 1273 K. However, many small steps appear along the curved interface; between these steps were short ð111ÞPd =ð0002Þ ZnO polar interfaces. To investigate the bonding nature of oxygen atoms near the {0002}ZnO polar interfaces, we collected EELS spectra. Figure 4a shows oxygen K-edge ELNES acquired from the ZnO precipitate interfaces. The red line in Fig. 4b indicates the spectrum acquired from the curved interface, while the blue line indicates that from the (111)Pd/(0002)ZnO interface. As a reference, the spectrum of bulk ZnO is shown in green. A pre-edge feature, indicated by the arrow in Fig. 4b, appears in the spectrum of the curved interface, but not in those acquired from the (111)Pd/(0002)ZnO interface or bulk ZnO. The
Microscopy, 2014, Vol. 63, No. 6
467
Pd
ZnO
polar interface was not completely lost at 1273 K. Another explanation is that the steps formed from the faceting transition of the curved interface, during air cooling of the sample after oxidation. Excess oxygen atoms in the matrix would have begun segregating at the curved interface during cooling, and many ð111ÞPd =ð0002Þ ZnO polar interface facets would have formed there. Further experimental and theoretical insight is required to confirm these explanations.
Concluding remarks Using HRTEM, EELS and CBED, we investigated the atomic and chemical structures of Pd/ZnO polar interfaces formed by internal oxidation of Pd–Zn alloys. At oxidation temperatures
