REGULAR ARTICLE. Preparation of Platinum Palladium Monoxide Pt 1-x Pd x OThinFilms

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1 REGULAR ARTICLE Journal of the Vacuum Society of Japan Vol. 53, No. 2 (2010) Preparation of Platinum Palladium Monoxide Pt 1-x Pd x OThinFilms Yuki IWAI* 1, Noboru MIURA* 1, Setsuko MATSUMOTO* 2, Ryotaro NAKANO* 1 and Hironaga MATSUMOTO* 1 * 1 Department of Electronics and Bioinformatics, Science and Technology, Meiji University, Higashimita, Tama-ku, Kawasaki , Japan * 2 Department of Physics, Science and Technology, Meiji University, Higashimita, Tama-ku, Kawasaki , Japan (Received August 31, 2009, Accepted December 28, 2009) Platinum palladium monoxide (Pt 1-x Pd x O) thin ˆlms with an arbitrary composition, where 0ÃxÃ1, were prepared by reactive cosputtering of platinum and palladium targets. The oxidation states of each atom of Pt and Pd were conˆrmed as that of PtO and PdO, respectively. Two infrared absorption peaks were found in the mid-infrared region and they exhibited linear peak shifts toward a lower wavenumber with increasing x. Thus, we completed successful preparation of Pt 1-x Pd x Othinˆlms with a homogeneous phase. The crystallinity of the Pt 1-x Pd x O thin ˆlms improved when the composition exceeded x=0.56. The preferred orientation changed continuously between x=0 (PtO)andx=1 (PdO). The optical bandgap decreased when x decreased from 1, and became unclear in a low Pd content region, including x=0 (PtO). 1. Introduction Platinum and palladium are extremely useful as catalysts in exhaust gas treatment and as electrode for microelectronics. In particular, palladium oxide is widely used owing to its high activity in the oxidation reaction of methane or carbon monoxide. Pt Pd alloys are used for improving this activity and preventing the degradation of the palladium catalyst. Recently, active cooperation between PdO and PtO was suggested as the reason for this improvement. 1) However, there is no report in the literature concerning the fundamental properties of composites of these oxides. Formation of a complete solid solution between PtO and PdO is expected because they are isostructural, i.e., their crystal structures belong to the same space group, and have equivalent atomic coordinates and almost the same lattice constants (º1.5 z). To investigate the fundamental properties of Pt 1-x Pd x O, a solid solution of monoxides PtO and PdO, we need samples of high quality with respect to their composition and crystallinity. PdO single crystals and thin ˆlms were prepared and their optical properties and electronic structures were precisely investigated 2 5) ; however, there are no reports on PtO single crystals, and thin ˆlms were indiscriminately deposited with other oxidation states, such as Pt, Pt 3 O 4,Pt 2 O 3, a PtO 2,and b PtO 2. This is because of the di culty of preparing high purity PtO thin ˆlm samples in a chemical bonding state. Recently, the importance of the substrate temperature was pointed out as a deposition condition of reactive sputtering 6).Inthisstudy,wepreparePt 1-x Pd x O thin ˆlms that are a mixed monoxide of PtO and PdO. We characterize them based on their chemical bonding state, crystallinity, and surface morphology, and use the infrared absorption spectra and optical bandgap to determine their fundamental optical properties. 2. Experimental Platinum palladium oxide thin ˆlms were prepared by multi-target radio frequency (RF) magnetron sputtering of metallic disc targets of platinum and palladium (4N purity), mm in diameter, in a side-sputtering con- ˆguration. The ˆlms were deposited onto fused silica substrates or Si(100) wafers set on a rotating carousel drum in the center of a vacuum chamber. Reactive sputtering was performed in a mixture of Ar and O 2 (6N purity) gas. The total gas ow rate was ˆxed as 20.0 sccm, and the O 2 ow ratio was maintained at 15 30z. The substrate temperature and discharge pressure during sputtering were controlled at C and 0.2 Pa, respectively. To investigate the ešect of the atomic composition ratio of Pt and Pd, the RF power applied to each platinum and palladium target was varied in the range 0 75 W. The deposition time was adjusted to produce a ˆlm thickness of nm. The atomic composition ratio of the thin ˆlms and the chemical bonding state of Pt and Pd were investigated by Rutherford backscattering spectroscopy (RBS) and X- ray photoelectron spectroscopy (XPS) using ESCALAB MKII (VG Scientiˆc) with monochromatic Al Ka radiation ( ev). The binding energies were referenced by the C 1 s core level (284.6 ev) because the samples showed a charging ešect. The crystallinity of the thin ˆlms was conˆrmed by X-ray dišraction (XRD) using the 4086A1 (Rigaku) with Cu-Ka radiation. The surface structure was observed by ˆeld emission scanning electron microscopy (FE-SEM) using the S 5200 (Hitachi). Near infrared (NIR) absorption spectra were collected using an FT-IR spectrometer, FTIR 8300 (Shimadzu). Ultraviolet-visible absorption spectra of the nm thick ˆlm sample were measured using a spectrophotometer UV 3100 (Shimadzu). 51 ) 109

2 3. Results and Discussion 3.1 Preparation of Pt 1-x Pd x Othinˆlms RBS measurements were performed to analyze the compositions of the thin ˆlms prepared by reactive cosputtering using platinum and palladium targets. Fig. 1 shows the relationship between the atomic composition x=pd/(pt+pd) of the thin ˆlms and RF power applied to each target. Fig. 1(a) shows x as a function of RF power applied to the palladium target, while that applied to the platinum target was ˆxed at 75 W. Manipulation of RF power applied to the palladium target from 0 to 75 W causes a linear change in x in the range Fig. 1(b) also shows x as a function of RF power applied to the platinum target while that applied to the palladium target was ˆxed at 75 W. Changing of RF power applied to the platinum target from 0 to 75 W causes a linear change in x in the range This RBS study showed that controlling the RF power applied to each platinum and palladium target enables the preparation of platinum palladium oxide thin ˆlms in the range 0ÃxÃ1. TheXPSspectrafromthecorelevelsofeachatom were obtained to conˆrm the chemical bonding states of Pt and Pd atoms in the platinum palladium oxide thin ˆlms. These spectra were deconvoluted using Gaussian curves on a baseline calculated by Shirley's method. Fig. 2(a) shows the spectra near the Pt 4f core levels. The x= 0 (platinum oxide) sample contains only one peak for Pt 4f 7/2, located at a binding energy of 72.8 ev. Because this value is in agreement with the reported value of 72.5 ev assigned to the bonding state of PtO 7), it is thought that the ˆlm is PtO with no other oxidation state. For the x= 1 (palladium oxide) thin ˆlm Fig. 2(b), which represents the spectra near the Pd 3d core levels, Pd 3d 5/2 peaks are found at and ev. Because these peaks were found in PdO, assigned with the binding energy of Pd 3d 5/2 (336.7 ev) and the satellite (339.4 ev) peak 8),itis thought that the ˆlm is PdO. As shown in Fig. 2(a), for the platinum palladium oxide thin ˆlms deposited by cosputtering, the peak strength from Pt 4f electrons decreases with increasing Pd content x, e.g., 0.38 and At the same x, the peak strengths from Pd 3d electrons increase, as shown in Fig. 2(b). No remarkable chemical shift is seen in any peak. This indicates that the platinum palladium oxide thin ˆlms prepared in this study were composed of PtO or PdO for the entire range of x. Since the thin ˆlms were monoxides, we refer to these thin ˆlms as Pt 1-x Pd x O, using the palladium content x. Figure 3 showsthexrdpatternsofpt 1-x Pd x Othin ˆlms with the dependence x. The dišraction peaks from PtO and PdO could arise at almost the same 2u because these compounds are isostructural (belonging to the same space group, having equivalent atomic coordinates, and almost the same lattice constant (a PtO and c PtO is about 1.1z and 0.05z larger, respectively 9,10) )). For the x=0 (PtO) thin ˆlms, a peak measured at 2u= 59 indicates that the ˆlm was tetragonal PtO with (103) or (200) preferred orientation 10).Forthex=1 (PdO) thin ˆlms, a peak measured at 2u=33.5 indicates that the Fig. 1 Atomic composition x=pd/(pt+pd) of platinum palladium oxide thin ˆlms as a function of the RF power. RF power was maintained at 75 W for each target: (a) platinum target and (b) palladium target. Fig. 2 XPS spectra of Pt 1-x Pd x O thin ˆlms near (a) Pt 4f and (b)pd3dlevels ) J. Vac. Soc. Jpn.

3 ˆlm was tetragonal PdO with (002) or (101) preferred orientation 9). With an increase in x from 0 to 0.23, the strength of the peak at 2u=59 showninthex=0 pattern becomes weaker, and that of the peak at 2u=33.5 shown in the x=1 pattern appears. Increasing x to 0.56 and 0.68 makes the peak at 2u=33.5 sharper, which suggests an improvement in crystallinity. It can be seen that the peak position of (002) or (101) dišraction shift toward a higher 2u angle when x increases from 0 to 0.23 andfrom0.68to1.thesepeakshiftsrepresentthatthe lattice constant of Pt 1-x Pd x O thin ˆlms changes monotonically between that of PtO and that of PdO depending on x. Therefore, Pt 1-x Pd x O thin ˆlms are thought to be a solid solution of PtO and PdO. The surface morphology, observed by FE-SEM, is shown in Fig. 4. Grains measuring a few tens of nm were foundin(a)x=0 (PtO). The grains grow in (b) x=0.41 andbecomethinwithnosizeincrease;theˆlmhassharp grain edges at (c) x=0.78. In (d) x=1, the grains become round and increase in size. The tendency of grain size to change withx agrees with the improvement of crystallinity in the XRD results. No simple changes are found in the grain shapes or boundaries of Pt 1-x Pd x O in the range of x from 0 to 1, suggesting an anomalous surface morphology of Pt 1-x Pd x O compared to that of PtO or PdO. 3.2 Optical properties of Pt 1-x Pd x Othinˆlms Figure 5(a) shows the infrared absorption spectra of Pt 1-x Pd x O thin ˆlms. For the x=1 (PdO) ˆlm, absorption peaks occur at 591 and 645 cm -1. According to references 11,12), these peaks are E 2 u and A 2u vibration modes, respectively. With decreasing x, for the x=0.81 and x=0.42 samples, the peaks indicate shifts to a higher wavenumber, and reach wavenumbers of 621 and 666 Fig. 3 XRD patterns of Pt 1-x Pd x Othinˆlms. Fig. 4 FE-SEM images of Pt 1-x Pd x Othinˆlms. Fig. 5 Absorbance spectra in infrared region of Pt 1-x Pd x Othin ˆlms. (a) Absorbance spectra and (b) a plot of peak wavenumbers. 53 ) 111

4 Fig. 6 Absorption spectra (a), (ahn) 2 hn plots (b) and optical bandgaps (c) of the Pt 1-x Pd x Othinˆlms. cm -1 in x=0 (PtO)ˆlm.Fig. 5(b) shows the plots of both peak positions, which exhibit linear changes against palladium content x. PtO and PdO have the same vibration modes because they belong to the same space group (D 9 4h). Similar peak shifts in report 13) are found with respect to other solid solutions. Therefore, Pt 1-x Pd x O thin ˆlms are thought to be a solid solution with a homogeneous phase, and this is good agreement with the result from XRD analysis. Figure 6(a) shows absorption spectra in NIR-UV region. No transparent region is found for x=0 (PtO) thin ˆlm prepared in this study, i.e., it has no absorption edge. The absorption slowly increases with increases in photon energy above 1.5 ev and peaks at 4.2 ev. Increasing x for x=0.38 and x=0.81 ˆlms, the absorption slope becomes steeper and a reduction of the absorption coe cient in low the photon energy region is found. For x=1 (PdO) ˆlm, the spectrum is characterized by a sharp peak at 2.4 ev, a weak peak and feature at 3.3 ev and 4.1 ev, respectively, and a signiˆcanttransparent region. These features agree with PdO thin ˆlms prepared by thermal oxidation studied by Nilsson and Shivaraman 3). Figure 6(b) shows (ahn) 2 as a function of hn (a: absorption coe cient, hn: photon energy). The optical bandgap for the x=1 (PdO) thin ˆlm is 2.11 ev, which is in good agreement with the reported value (2.13 ev) of PdO ˆlm prepared by thermal oxidation from Rey et al 5). With decreasing x for 0.81 and 0.38 samples, the optical bandgap narrows and the absorption tail moves to the low-photon-energy side. The optical bandgaps are plotted in Fig. 6(c) as a function of x. We can see a decrease in a high Pd content region, and almost constant values below the region. For x=0 andlowx samples, it was di cult to determine the optical bandgaps. An experimental optical bandgap in PtO has not been found in previous studies 14,15). 4. Conclusion Platinum palladium monoxide (Pt 1-x Pd x O) thin ˆlms of arbitrary composition, where 0ÃxÃ1, were prepared successfully by reactive cosputtering platinum and palladium targets. The oxidation states of each atom of Pt and Pd were conˆrmed as that of PtO and PdO, respectively. XRD revealed the preferred orientation of x=0 (PtO) ˆlms to be (103) or (200), and x=1 (PdO)ˆlmsto be (002) or (101). Two infrared absorption peaks, corresponding to the lattice vibration mode of E 2 u and A 2u, were observed. They exhibited linear peak shifts with x, which implies the formation of Pt 1-x Pd x O crystallites as a solid solution. The crystallinity of the Pt 1-x Pd x Othin ˆlms improved with a composition of x over The preferred orientation changed continuously between x= 0 and x=1. The optical bandgap decreased drastically when x decreased from 1 to 0.81, and became unclear at x=0 (PtO) and in the region of low Pd content. Acknowledgements The present work was partly performed at the UVSOR facility under the Joint Studies Program of the Institute for Molecular Science. References 1) A. Maione, F. Andráe and P. Ruiz: Appl. Catal. B: Environ., 75 (2007) ) J. Vac. Soc. Jpn.

5 2) R. Ahuja, S. Auluck, B. Johansson and M. Khan: Phys. Rev. B, 50 (1994) ) P. O. Nilsson and M. S. Shivaraman: J. Phys. C, 12 (1979) ) W. Weber, J. Remillard, J. McBride and D. Aspnes: Phys. Rev. B, 46 (1992) ) E. Rey, M. Kamal, R. Miles and B. Royce: J. Mater. Sci., 13 (1978) ) K. Kuribayashi and S. Kitamura: Thin Solid Films, 400 (2001) ) Y. Abe, H. Yanagisawa and K. Sasaki: Jpn. J. Appl. Phys., 37 (1998) ) T. Pillo, R. Zimmermann, P. Steiner and S. Hufner: J. Phys.: Cond. Matt., 9 (1997) ) JCPDS XRD Database, Cards: ) J. McBride, G. Graham, C. Peters and W. Weber: J. Appl. Phys., 69 (1991) ) G.Kliche:Z.Naturforsch.A,44 (1989) ) G. Kliche: Infrared Physics, 25 (1985) ) C. Bundesmann, A. Rahm, M. Lorenz, M. Grundmann and M. Schubert: J. Appl. Phys., 99 (2006) ) H. NeŠ, S. Henkel, E. Hartmannsgruber, E. Steinbeiss, W. Michalke, K. Steenbeck and H. Schmidt: J. Appl. Phys., 79 (1996) ) C.R.Aita:J.Appl.Phys.,58 (1985) ) 113

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