X-Ray Photoelectron Spectroscopic Studies of Palladium Oxides and the Palladium-Oxygen Electrode K. S. Kim, A. F. Gossmann, and Nicholas Winograd Department of Chemistry, Purdue University, West Latayette, Ind. 4 7906
The ESCA technique has been employed to characterize various palladium-oxygen species on the surface of oxidized palladium metal. Both oxygen-chemisorbed palladium atoms (PdOad,) and PdO are observed on metal substrates exposed to air at 600 to 900 "C. These results have been applied to studying electrochemically oxidized palladium electrodes in 1N HzS04. Both PdO and PdOz are observed at potentials beginning at -I-0.90 V vs. N.H.E. In addition, since the mean escape depth of the photoelectron is on the order of 10 A, estimates of oxide film thicknesses can be made. Electrodes oxidized at 0.9 V had a coating about 5 A thick whereas at 1.7 V, the thickness increased to greater than 40 A. By examining the peak areas of the Pd spectra and the oxygen spectra, the presence of excess oxygen as adsorbed and H20, hydrated PdO, PdOz, and Pd(0H)z or Pd(OH)4 was clearly indicated.
The last decade has witnessed the emergence of several supplementary tools which aid the electrochemist in interpreting often-times hopelessly complicated currentvoltage curves. Extensive development of optical spectroscopy, X-ray and electron diffraction, electron microscopy, field-ion microscopy, and radiotracer techniques substantiate this claim. Some approaches are valuable because they can be used in situ, that is, to monitor processes in the electrochemical cell while they are occurring. Others, particularly those involving X-ray or electron energy analysis, must be used as analytical tools after the electrochemical process is complete. These analyses are generally restricted to a chemical identification of species remaining in solution or to a n elemental analysis of material remaining on the electrode. The technique of X-ray Photoelectron Spectroscopy ( X P E S or ESCA) offers a n ideal approach for the analysis of surface films remaining on electrodes after the electrochemical perturbation is complete. Although the technique cannot be applied in situ, information related to chemical structure and thickness of these films can be obtained in a straightforward manner. We first illustrated this approach to analysis by successfully identifying the two oxides of Pt ( P t O and PtOz) ( 2 ) on the surface of a platinum electrode oxidized in aqueous acidic solutions. The chemical identification of certain hydrates has also been confirmed for the Pb/PbOz electrode ( 2 ) . In addition, Hulett and coworkers (3) have studied the passivation of some copper-nickel alloys and have confirmed the presence of NiO as the primary surface species. The purpose of this paper is to describe the initial results we have obtained on the analysis of several palladium-oxygen species and to describe the application of (1) K. S. Kim, N. Winograd, and R. E. Davis, J . Amer. Chem. SOC.,93, 6296 (1971). ( 2 ) K. S. Kim, T. J. O'Leary, and N. Winograd, Anal. Chem., 45, 2214 (1973). (3) L. D. Hulett. A. L. Bacarella, L. LiDonnici, and J. C. Griess, J. Electron Spectrosc., 1 , 169 (1972/73).
these results to the analysis of anodic oxide films formed on the palladium electrode, oxidized in sulfuric acid media. The ESCA d a t a indicate the presence of both PdO and PdOz, as predicted by previous electrochemical irivestigations. The ESCA technique is also used for estimating the approximate thickness of these surface films since the mean escape depth for 1000-eV electrons is on the order of 10 A ( 4 , 5 ) . The possible information obtained from ESCA and the value of this technique to electrochemists will also be discussed.
EXPERIMENTAL Apparatus. Spectra were recorded on a Hewlett-Packard 5950 A ESCA spectrometer using monochromatic A1 Kcvl,~X-rays obtaiqed from a quartz-crystal disperser. Binding energies were calibrated to the Au 4f7:z level of evaporated Au a t 84.0 eV or the C Is level of graphite a t 284.4 eV. The normal pressure in the spectrometer chamTorr as read on the ion pump gauge. ber was 5 x Electrochemical measurements were performed using a potentiostat of standard design built from operational amplifiers. Reagents. Palladium foil (99.9%) was obtained from Alpha Inorganics and used without further purification. Procedure. Electrochemical measurements were performed in 1N H2SO4 supporting electrolyte a t 25 "C. Electrode potentials were measured with respect to the saturated calomel electrode but are reported with respect to the normal hydrogen electrode. Solutions were degassed with argon prior to the oxidations, although no attempt was made to sweep out any oxygen which may have formed during the experiment. After performing the oxidations, the electrodes were removed from the solution, rinsed carefully in distilled water, and immediately placed in the ESCA instrument. Spectra were deconvoluted using a Dupont 310 curve resolver. The location and the full width a t half maximum (FWHM) for a species was first determined using the spectrum of a pure sample. The location and FWHM of products which were not obtained as pure species were adjusted until the best fit was obtained. Symmetric gaussian shapes were used in all cases. Binding energies for identical samples were, in general, reproducible to within *O.l eV. RESULTS AND DISCUSSION The ESCA spectrum of Pd foil abrased with Sic in air is shown in Figures l a and l b , and the binding energy values are summarized in Table I. The 3ds/z and 3d3/2 peaks always show a slight skewness toward the high binding energy side. From previous investigations ( 2 , 2, 6, 7 ) , this behavior is normally attributed to chemisorbed oxygen, although for palladium this cannot be verified by , ~ at the 0 1s peak due to the presence of the Pd 3 ~ 3 line 531.4 eV (Figure 1 6 ) . This value is very close to the value expected for the 0 1s electron from chemisorbed oxygen atoms. Evaporation of Pd foil in the spectrometer cham( 4 ) T. A . Carlson and G. E. McGuire, J. Electron Spectrosc., 1 , 161 (1972/73), and references cited therein. ( 5 ) M . L. Trang and G. K. Wehner, J . Appl. Phys., 44, 1534 (1973). (6) K. S. Kim and N. Winograd, Chem. Phys. Left., 19, 209 (1973). (7) K. S. Kim and R . E. Davis, J. Electron Spectrosc., 1, 251 (197.2,' 731.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
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7
I
4,'
I
I
Table I. B i n d i n g Energies (eV) of Palladium 3dsi2,3pytr,and 0 1s E l e c t r o n s for Various Palladium-Oxygen Species. Species
Pd 3dw
Pd 3piii
0 1s
Pd PdOsba PdO PdOz
335 .o 335.6 336.3 337.9
531.4 N.R.b 532.7 534.3
... N.R. 529.3 N.R.
a Binding energies are calibrated to the Au 4f7/2electron peak at 84.0 eV. Not resolvable.
E(z
I
Table 11. Relative P e a k Areas in t h e Palladium 3p,/, and Oxygen 1s B i n d i n g Energy Region f o r Various Palladium-Oxygen Samples , , - . . -L ' - -JL
I
5430
3390
3350 BINDING
5330
52190
Figure 1. X-Ray photoelectron spectra of ( a ) Pd 3 d 5 / 2 ~ ~and 2 ( b ) 0 1s and Pd 3p3i2 electrons for Pd foil cleaned by abrasing
with Sic. In (c) and (d),the foil was heated in air at 900 "C for several hours and in (e) and ( 0 the foil was heated in air at 600 "C for several hours. See Table I for binding energies corrected for charging. In ( d ) the highest binding energy peak consists of , ~ for both PdO and PdOad,, since it is impossible the 3 ~ 3 peak
to resolve it further ber a t 10--6 Torr followed by bombardment with 400 eV Ar' produced symmetric Pd 3d peaks. Various pretreatments of the Pd foil were tested. Washing with hot, concentrated HzSO4 yielded results essentially indistinguishable from those given in Figures l a and b. Treatment with concentrated " 0 3 , however, produced a surface layer of PdO. Electrochemical pretreatments resulting from cycling the potential in 1N HzS04 also indicated that the surface was completely free of phase oxide but, as is shown in Figures l a and b, it is partially covered with a chemisorbed oxygen type species which may be attributed either to oxygen atoms, water, or contamination from the supporting electrolyte. The origin of these species is as yet open to question, although surface studies on Pd evaporated in the spectrometer followed by argon ion surface cleaning and residual gas analysis are presently under way to further elucidate this point. The spectrum of the 3d5/2,3,2 electrons for Pd foil heated in air to 900 "C is shown in Figure I C . This treatment is known to produce PdO on the metal surface, however, since this temperature is near the evaporation temperaa mixture of PdO, PdOads, and ture (8) of PdO (850 T), Pd metal can clearly be discerned. When heating a t slightly lower temperatures, the surface is completely covered with PdO (Figures l e and f ) and the presence of PdOad, and Pd metal is obscured. A small amount of surface charging is observed for PdO, and the actual binding energy value for the Pd 3d5/2 peak can vary from 336.3 to 336.9 eV depending on the thickness and morphology of the oxide layer. The oxide film in Figures I C and d, for example, is equivalent to roughly a monolayer of material on the surface, as determined from a n approximate escape depth of 1000 eV electrons through PdO of about 15 8, ( 4 ) . The magnitude of the charging (-0.6 eV) plus the distinct appearance of PdOad, strongly suggest that the oxide has clustered into small islands, leaving large sections of the Pd metal still exposed. The PdO films shown in Figures I C and d are insoluble in 1N HzS04 since this treat(8) J. C . Chaston, Platinum Metals Rev., 9, 126 (1965) 198
__
Area, %b
52%
ENERGY ( e V )
Sample4
3PaP Pd(rneta1)
3pap Pd-oxygen
0 Is All oxygencontaining components
Figure lf Figure Id Figure 2b Figure 2d Figure 36 Figure 3d
0 53 49 17 11 0
60 27 27 28 23 26
40 20 24 54 66 74
Ratio'
0.7 0.7 0.9 1.9 2.9 2.8
a Samples listed are those given in the designated Figure. Treatment conditions are given in the Figure caption. See text for details of the deconvolution procesa. Calculated by dividing the 0 1s peak area for all oxygen species by the Pdw p a k area for all Pd-oxygen species. Values are approximate and are probably valid to =kZO%.
*
ment does not change the 3d5/2 electron peak area ratio for PdO to Pd. The Pd 3p3,2 and 0 1s spectra for the above samples are shown in Figure 1, and the binding energy values for the known species are summarized in Table I. Not all peaks could be definitively assigned. For example, in Figure If, a peak appears a t 530.5 eV which we do not associate with pure PdO. Speculatively, we guess that this peak arises from adsorbed water or oxygen chemisorbed on the PdO surface. In the other spectra (Figures 2 and 3), the lines are very broad and the number of peaks so numerous that it is possible to identify only the most obvious peaks. From Figure I f , the coincidental interference of the Pd 3p and 0 1s peak is clearly visible. Although this interference complicates a complete description of peaks observed in this energy region, it does provide a convenient approach to comparing the relative areas of the peaks arising from Pd and oxygen species. In Table 11, we have compared the area ratio of total peak areas between the Pd 3 ~ 3 ~and 2 0 l s peaks. For the spectrum in Figure If, for example, the relative areas for the P d 3~312 peak and the 0 Is peak of P d O are 60 and 4070,respectively. We have not included the 530.5-eV peak in this estimate since it probably arises from excess oxygen (chemisorbed oxygen and/or water) and therefore does not contribute to the Pd peaks. For the sample in Figure Id, the areas are calculated by curve fitting the Pd 3~312peaks in accord with the Pd 3d peaks and assuming any leftover area is attributed to the 0 1s signal. The ratios for Figure Id and Figure I f are quite close, as expected for these samples. We shall use this ratio to detect the presence of excess oxygen species on the Pd-oxygen samples generated electrochemically. The electrochemical behavior of the Pd-oxygen electrode has been studied by a number of workers by moni-
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 2 , F E B R U A R Y 1974
C I
I
A
3420
3380
3340"
533 0
5290
5250
E N E R G Y ieV1
BINDING
Figure 2. X-Ray photoelectron spectra of ( a ) Pd 3d5,2,3/2and ( b ) 0 1s and Pd 3 ~ 3 electrons ;~ for a Pd foil oxidized at +0.90 V for 1000 sec in 1 N H2S04. In (c) and ( d ) , a foil was oxidized at 1.28 V for 60 sec
+
toring the open-circuit potential of the palladium electrode after application of a controlled current and studying potential variations under various solution conditions ( 8 1 4 ) . The rest potentials for t h e PdO/Pd and PdOZ/Pd couples are generally considered to be about 870 and 1470 mV, respectively, in oxygen saturated 1N H2S04. Hoare (9) indicates that since the 870-mV potential depends on the oxygen pressure and does not depend on pH, the electrode reaction must be composed of a sequence of reactions. His scheme in the low potential region ( > -800 mV) consists of the formation of PdO a t 870 mV Pd H20 PdO 2H' 2e(1)
+
+
+
-
+
and the corrosion of t h e P d electrode Pd
Pd2+
+
(2)
2e-
followed by the formation of PdO2 on the electrode Surface
0,
+
Pd2+
+
2e-
---t
(3)
PdO,
The PdO species can then be formed through the slow decomposition of PdOz
-
+
PdO, PdO x02 (4) An equilibrating system of reactions then is set u p yielding a few monolayers of PdO and a trace amount of PdO2 on the electrode surface. The corrosion of the Pd electrode has recently been studied as a function of potential using radiotracer techniques (10) and the results are a t least consistent with reaction 2 . Hoare's proposed mechanism presents a n interesting pathway for the production of PdO2 a t potentials far below its equilibrium electrochemical potential of 1470 mV. At the potential region above 1470 mV, in addition to Equations 1-4, P d is oxidized directly to PdOz Pd
+
2H,O
+
PdO,
+
4H+
+
4e-
(5)
The ESCA spectra for all Pd electrodes oxidized below 800 mV were essentially identical to those given in Figures l a and b, indicating a complete lack of any phase oxide J. P. Hoare, J. Electrochem. SOC,111,610 (1964). J. F. Llopis, J. M. Gamboa, and L. Viclori, Electrochim Acta, 17, 2225 (1972) K . J. Vetter and D. Berndt, 2. €/ektrochem., 62,378 (1958). A . Hickling and G. G. Vrjosek, Trans. Faraday SOC.,57, 123 (1961). J. A . V. Butler and G. Drever, Trans. Faraday SOC., 32, 427 (1936). S . E. S . El Wakkad and A . M. Shams El Din, J. Chem. SOC., 1954, 3094.
I
1
/ /
342.0
3380
3340 "
5330
5200
5250
BINDING ENERGY ( e v )
Figure 3. X-Ray photoeiectron spectra of ( a ) Pd 3ds:2,3;2 and ( b ) 0 1s and Pd 3p312 electrons for a Pd foil exposed to a potential increasing from f O . 0 V to f 1 . 7 1 V at 40 mV/sec. In (c)
and ( d ) ,a foil was exposed to a constant potential of 1.71 V for 30 sec formation. The major source of current flow in the cyclic voltammogram in this potential region is undoubtedly due to the direct corrosion of the electrode (reaction 2 ) . This conclusion is consistent with the radiochemical observation that P d corrodes before the onset of phase oxide a t 870 m b ( 1 0 ) . The spectra for a Pd electrode which has been oxidized in 1N HzS04 at 0.90 V for 1000 sec are shown in Figures 2a and b. The 3d5,2,3:2 peaks clearly indicate the existence of a t least three forms of Pd: the pure metal a t 335 eV. the phase oxide PdO a t 336.3 eV, and another form occurring a t still a higher binding energy of 337.9 eV. The chemical shift for the PdO species (+1.3 eV L'S. Pd metal) corresponds to the value for the chemically produced PdO which shows no charging (336.3 eV). Since the area ratios for the 3d5 2 peak of PdO to Pd indicate a surface concentration of about 2 monolayer equivalents of PdO and since the charging effect is minimal, we suspect a rather smooth morphological distribution of the oxide about the surface. It is most tempting to assign the high binding energy form to PdOn. We have unsuccessfully attempted to prepare PdOz chemically from the available literature preparations (15, 16) to confirm this assignment. The dioxide, however, is unstable in the anhydrous form ( 1 5 ) and apparently decomposes to the observed product, PdO, after exposure to the high vacuum conditions in the spectrometer. By comparing the magnitude of the observed binding energies for Pd, PdO, and the high binding energy form in Figure 2a (Table I ) to those observed for Pt, PtO, and P t 0 2 [71.1, 73.7, and 74.5 eV for 4f-i:~electrons ( I ) ] , the evidence for a PdO2 type species on the electrode anodized a t +0.90 V is very strong. It is interesting that this form of PdO2 is quite stable in the ESCA instrument for several hours; this indicates that the environment of the Pd4+ is critical to its lifetime. Clearly, differences exist between the compounds we attempted to prepare chemically and those we prepared electrochemically but further investigation is needed to pin down these differences explicitly. These results strongly support the conclusions of Hoare (9) which were based on indirect electrochemical evidence. The spectra of the Pd electrode oxidized a t 1.28 V (Fig(15) N. V . Sidgwick, "Chemical Elements and their Compounds," Oxford University Press, 1950, p 1575. (16) H. Remy, "Treatise on Inorganic Chemistry," Elsevier, Amsterdam, 1956, p 341.
ANALYTiCAL CHEMISTRY, VOL. 46, NO. 2 , F E B R U A R Y 1974
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ures 2c and d ) for 60 sec also show three forms of P d : P d metal, PdO, and a small amount of the PdO2 species. Again, t h e charging effect of P d O is minimal for this film estimated to be a n equivalent of 4 or 5 layers thick, indicating the smooth rather than clumped nature of the film. Oxidation a t still higher potentials (Figures 3a-d) yields films of thicker oxide layers using shorter oxidation times. At 1.71 V, the ratio of P d O to PdOz always remains much larger t h a n expected since t h e electrochemical d a t a (reaction 5 ) predict t h a t P d 0 2 is t h e principal product in this potential region. This result could be due to either the decomposition of PdOz on the surface or to spontaneous reduction of PdO2 as the electrode is removed from the solution. Kote that in Figures 3c and d, the presence of Pd metal is completely obscured, indicating t h a t the thickness of the oxide layer is a t least 40 A. I n addition, the oxide film produced by sweeping the potential to 1.71 V rather than stepping to 1.71 V contains a much lower amount of PdO2. This effect may be partially explained by the relatively shorter time the electrode is exposed to potentials above 1.47 V. The 0 Is spectra for all these Pd-oxygen electrode systems are difficult to quantitatively interpret because of the interference from the P d 3 ~ 3 levels , ~ of P d , PdO, and PdO2. For all the electrochemically generated Pd-oxygen species, however, the area ratios between t h e Pd 3 ~ 3 and , ~ 0 Is peaks exceeded those for the chemically formed PdO (Table 11). These ratios were calculated by initially superimposing Pd 3 ~ 3 peaks, , ~ whose positions were determined using the corresponding Pd 3d peaks, on the composite Pd 3p3 2 - 0 Is spectra to make a “best” fit. We did not attempt to deconvolute the 0 IS peaks but rather estimated the peak area contribution for the sum of all the oxygen species as given by the difference between the Pd 3p3 2 assignments and the entire spectral curve. The fact t h a t this ratio varies u p to 4.2 times the chemically
formed P d O depending on the electrochemical potential is strong evidence for large quantities of excess oxygen in the form of adsorbed water, hydrated PdO or PdO2, or the corresponding hydroxide P d (OH)n or P d ( 0 H ) r .
CONCLUSIONS The ESCA technique is a valuable aid in directly identifying t h e chemical nature of species produced a t electrode surfaces. I t is clear t h a t many fundamental questions regarding electrode reactions can be quickly a n swered. For example, the spectrum in Figure 2a conclusively shows the presence of a phase oxide rather than chemisorbed oxygen atoms and also indicates the presence of PdOz, lending strong support to Hoare’s mechanism. We feel t h a t , in addition to these direct assignments, future studies of t h e 0 1s region using a variety of electrode materials will allow characterization of excess oxygen species such as OH-, adsorbed water, and bulk hydrates. The approach should also be valuable in identification of organic and inorganic films which often foil electrode behavior. Results of this kind can obviously be of great aid in proposing the electrochemical mechanism.
ACKNOWLEDGMENT The invaluable assistance of Jon Amy and William Baitinger in spending countless hours keeping the instrument in excellent condition arrd the inspiration from their many helpful discussions are gratefully acknowledged. Received for review July 5 , 1973. Accepted August 22, 1973. The authors wish to thank the National Science Foundation (Grant No. GP-37017X) and the Air Force Office of Scientific Research (Grant No. AFOSR-72-2238) for financial support. The authors also appreciate the equipment grant from the Xational Science Foundation which allowed purchase of the ESCA instrument.
Correction of Inner Filter Effects in Fluorescence Spectrometry V. Alan Mode and D. H. Sisson Lawrence Livermore Laboratory, University of California, Livermore, Calif. 94550
Quantum efficiencies can be easily and rapidly determined by reference to a standard solution of known quantum yield using a quantum counter and front-surface, excitation-emission techniques. Because differences in the optical densities of the sample and reference solutions will result in the incident light exciting different volumes of solution, it is necessary to correct the data for the nonlinear losses of light out the sides and rear of the cell. I t has been possible to derive an analytic expression to correct for these losses which provides consistent results for a wide range of optical densities.
The determination of photoluminescent quantum yields has not been a simple task. The techniques and problems associated with these measurements have recently been summarized in a n extensive review article ( I ) . In order to make these measurements of more practical use, several methods have been developed t h a t employ “standard” 200
materials and comparison or ratio measurements. One of the most popular of these methods was developed by Bowen ( 2 ) and is based on the original Vavilov technique ( 3 ) for determining absolute fluorescent quantum efficiencies. The fluorescent emission from the sample cell is caused to fall on a material which will absorb quanta over a broad spectral range, but which emits radiation with a fixed spectral distribution irrespective of the energy of absorbed quanta. This fluorescent screen when used with a phototube has been termed a quantum counter ( 2 ) . This quantum counter effectively integrates the total fluorescent emission from the sample cell. The quantum counter’s fluorescent emission is directly proportional to the total energy absorbed, independent of the spectral distribution of the exciting light. Because the photodetector views only the fixed spectral distribution of the quantum (1) J. N. Demas and G. A . Crosby,J. Phys. Chern.. 75,991 (1971). ( 2 ) E. J. Bowen, R o c . RoyaiSoc., Ser. A , 154a. 349 (1936). (3) S.I . Vavilov. 2. P h y s . . 22, 266 (1924).
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