X-Ray Photoelectron Spectroscopic Studies of Cadmium- and SiIver-Oxyg en Surf aces J. S. Hammond, S. W. Gaarenstroom, and Nicholas Winograd Department of Chemistry, Purdue University, West Lafayette, Ind. 47907
The surface chemistry of the Cd- and Ag-oxygen systems has been studied by X-ray photoelectron spectroscopy (XPS or ESCA) using both temperature changes and ion bombardment techniques as methods of Inducing structural changes. For the Cd system, the 0 1s binding energies for CdO, Cd(OH)2, CdC03, and Cd02 have been Characterized and have been used to follow the decomposltion of these species as a function of temperature. For oxygen-exposed evaporated flims of Cd, two distinct oxygen phases were noted with one being specifically identified as CdO. For the Ag system, the 0 I s binding energies of Ago, Ag20, and Ag2C03 were reported. On the oxides prepared In air, a significant carbonate contaminant was noted which complicated the Interpretation of the 0 1s spectra. For oxygen-exposed evaporated Ag films, no oxide phases were noted even at high oxygen exposures although a broad, structureless background, probably corresponding to absorbed oxygen, was observed.
X-Ray photoelectron spectroscopy (XPS or ESCA) has been extensively used for structural studies of wide classes of inorganic and organic molecules (1, 2). Because of the limited mean escape depth of the detected electron, generally on the order of five atomic layers ( 3 ) ,the technique is also becoming prominent in the field of surface analysis. T h e study of metal oxides and the corresponding oxidation of metal films by ESCA has been initiated by many research groups (4-9). Our research efforts have also focused on several metal-oxygen systems with an emphasis on elucidating the contrast between surface and bulk characteristics (10-16). T h e use of binding energy shifts and peak areas to correlate the stoichiometry of oxidized metal surfaces with known compounds has been successful in many of these systems (10-15). In this work, we have examined the passivation of evaporated metal films of cadmium and silver by oxygen and air exposure. Many aspects of the specific nature of the complex surface chemistry for these systems have been elucidated by spectral comparison with known compounds. For example, by the use of in situ chemical manipulation of these species to circumvent the problems of differential sample charging, and hydration and carbonation effects, we have characterized the XPS spectra for five different cadmium-oxygen species and five different silver-oxygen species observed as layers on pure metal films or as bulk compounds. This data base is then employed to analytically monitor further surface chemical alterations induced by a variety of sample pretreatments. T h e Cd and Ag systems are of particular interest since the usual direct correlation of core electron binding energy chemical shift with oxidation state shows many anomalies. For example, AgaO, Ago (8), CdO, and CdOz (17) all have negative chemical shifts with respect to the metal. In addition, the surface properties of these two metal-oxygen systems have many important uses in electrical components, corrosion inhibitors, oxidative and rearrangement catalysts, and in solid state batteries (18).
EXPERIMENTAL Apparatus. All spectra were recorded on a Hewlett-Packard 5950A ESCA spectrometer using monochromatic A1 K c Y X-rays. ~,~ Our instrument is equipped with a cross probe for in situ evaporation of metals and a variable temperature sample probe. Other accessories include a sputter ion gun (Physical Electronics, Inc.), a gas manifold with molecular leak valve into the sample handling chamber, a nude Bayard-Alpert gauge, and a quadrupole mass spectrometer (Finnegan Model 400) for residual gas analysis. After some instrumental modifications, work was done with the sample chamber at 5 X low9Torr and a spectrometer pressure of 1 X lo-$ Torr. The modifications include the addition of a titanium sublimination pump, two-stage differential ion pumping of the sample probe introduction chamber, a four-day instrument bake-out at 150 "C, and enclosure of the exposed part of the sample probe in a controlled atmosphere box (Vacuum Atmospheres Model H-43-6). This last modification also allows us to prepare samples in an atmosphere of less than 5 ppm oxygen and water vapor and insert them into the instrument without air exposure. Further details of our experimental apparatus are given elsewhere (19). Reagents. The CdO (J. T. Baker Co.), AgzO (Allied Chemical), and A g o (Alfa Inorganic) were used without purification. Ag foil purity was 99.99% and Cd foil (Alfa Inorganic) purity was 99.999%. Cd(OH)2 was prepared by the methods of Fahim (20) and Denisov (21). CdOz was made by the method of Hoffman, Ropp, and Mooney (22). The CdC03 and AgzC03 were prepared by precipitation of K2C03 with Cd(NO& and AgN03 respectively, then dried at 110 "C for 12 hours. The 0 2 for gas exposure (Matheson) was 99.99% pure and was injected into the spectrometer from an allmetal gas manifold through a molecular leak valve. Procedures. Since ESCA is a surface technique, sample cleaning is important. Metal films evaporated at lod8 Torr were sputter ion etched or examined directly without cleaning. The commercial metal oxides were found to have heavy surface concentrations of moisture and various adsorbed carbon species. These oxides cannot be cleaned by sputter ion etching since ion bombardment causes the oxides to reduce (19). The most satisfactory methods of surface cleaning are heat treatment in the inert atmosphere box or in situ heating with the variable temperature sample probe. Most of the compounds examined are insulators and, therefore, have a significant propensity to develop a surface charge during X-ray irradiation. Several procedures are followed to compensate for the surface charge in order to obtain consistent binding energy measurements. 1) Powered samples are prepared by burnishing the sample onto the surface of a gold coated sample blank with a sapphire bead. Using this procedure, a thin layer of sample in good electrical contact with the metal blank results. This minimizes sample charging or makes the sample charge more uniformly. Oxides grown on active metal surfaces are generally thin enough so that electrical equilibrium is maintained. 2) The order of magnitude of the charging effect can be estimated by exposing the sample to nearly zero kinetic energy electrons from an electron flood gun. If charging exists, the peak positions will vary slightly with the electron flux due to the presence of some non-zero kinetic energy electrons. If electrical equilibrium is maintained, however, the peak position is invariant to changes in the electron flux. Although this approach is not particularly useful for measurement of binding energies to better than f0.5 eV accuracy, i t does prevent the peaks from shifting during analysis, minimizes differential charging, and improves the precision of the measurements to, in most cases, better than f 0 . 2 eV. 3) In some samples, the evaporation of a thin discontinuous gold film serves as a useful reference (23). Since the gold is isolated from the spectrometer by the sample, any charging in the sample will also appear in the binding energy of the gold peak if the gold
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2193
L
Table I. X-Ray Photoelectron Binding Energies (eV) for the Cd-Oxygen Systema Cd % / 2 ,
eV
Cd'b' CdO'C' Cd(OH),"' Cd02(C) CdC03(') O(c) (unknown species)
404.8 404.0 404.6 403.4 404.9
...
0 Is, eV
C Is, eV
A
...
*.. ...
...
528.6 530.7 530.3 531.2 530.9
124.6
. . . 126.1 . . . 126.9 289.1 126.3 ... ...
aReferenced to Au 4f7:2 level at 83.8 eV. bEvaporated from tungsten filament in spectrometer whose base pressure was 5 x 10-9 Torr. See text for method of sample preparation.
530.0
33.0 BINDING
527.0
ENERGY (eV)
Flgure 1. 0 1s electron spectra of (a)Cd(OH)2; ( b )Cd(OH)* heated in situ at 140 O C for 36 minutes; (c)sample in ( b )heated at 250 O C for 30 minutes; (d)sample in (c) heated at 350 O C for 20 minutes; (e) Cd(OH)2 exposed to 400 eV Ar+ ion-bombardment(4 PA, 3 min.)
particles are in electrical equilibrium with the surface. The observed sample binding energies are then referenced t o the Au 4f7/2 peak a t 83.8 eV. Several difficulties with this procedure have been discussed including chemical alterations which occur during evaporation ( 2 4 ) ,the dependence of the corrected binding energies on the amount of gold evaporated (25),and the influence of possible matrix effects (26).We have used this technique with great care, therefore, and have not relied solely on this approach to compensate for charging. 4) Samples subject to charging used for in situ thermal and oxygen exposure are referenced using the 0 1s to Cd 3d5/2 or 0 1s to Ag 3d6/2 binding energy separations (A values) to identify a particular chemical species. Spectra were deconvoluted with a DuPont 310 curve resolver using symmetric gaussian peak shapes. This procedure is employed only as a rough guide to characterizing peak shapes and should not be construed as being mathematically rigorous or unique. RESULTS AND DISCUSSION Cadmium. The binding energies of Cd 3d5/2 and 0 1s electrons for various cadmium-oxygen species are given in Table I. The binding energies are calibrated to the gold 4f7j2 peak at 83.8 eV using a combination of the charging correction techniques discussed above. Note that the variation of the Cd 3d5/2 values is only 1.4 eV while the 0 1s values shift by as much as 2.3 eV. We have, therefore, concentrated our efforts on monitoring the 0 1s spectra rather than the Cd 3d5/2 spectra. The spectra of Cd(OH)2 and various in situ treatments of Cd(OH)2 are found in Figure 1. The spectra lb, IC, and I d show the thermal dehydration of Cd(OH)2 to CdO at 140, 250, and 350 OC, respectively. The decomposition spectra reveal a 2.7-eV separation between the OH- and 02-0 1s peaks in comparison with a 2.1-eV separation between the 0 1s peaks for pure Cd(OH)2 and CdO. This result is indicative of a small amount of differential charging between 2194
the Cd(OH)1 phase which is an insulating phase and the CdO which is a semiconductor and, therefore, should show insignificant charging. The change of the 0 1s intensity ratio of Cd(OH)2 a t 530.7 eV to CdO a t 528.6 eV with the in situ temperature treatment qualitatively parallels the thermogravimetric analysis of Fahim and Abd El-Salaam (20) and Low and Kamel (27). Because the ESCA results reflect surface composition in contrast to thermogravimetric analyses which reflect bulk composition, and because the thermogravimetric analyses were performed a t atmospheric pressure, no quantitative correlation is given. However, the ESCA observation that Cd(OH)2 surface dehydration is a discrete process involving the direct transformation from Cd(OH)2 to CdO in the range of 140 and 350 "C contrasts the assertion of Low and Kamel that the stoichiometry of the bulk material changes from Cd0.1.65 HzO to Cd00.80 in five steps. Our results show the Cd(OH)2 sample appears to dehydrate and change from a Cd(OH)2 species to a separate CdO lattice phase since we find only two 0 l s peaks which maintain a constant separation indicative of two distinct phases rather than a gradual shift of the 0 1s signal from a binding energy characteristic of OH- to a binding energy characteristic of 02-.The differential charging of the Cd(OH)2 also supports the assertion that the Cd(OH)2 and CdO form two distinct crystal phases with a discrete electrical conductivity boundary between the two phases. The use of Ar+ bombardment on Cd(OH)2 also produces oxide formation similar to in situ heating with the variable temperature sample probe. After three minutes exposure of pure Cd(OH)2 to 4-MAAr+ flux a t 400 eV kinetic energy, the CdO species accounts for 20% of the intensity of total oxygen containing species observed in the oxygen spectrum (Figure le). The Ar+ ion is thought to participate in sputtering of the surface species and to generate localized thermal spikes in the sample derived from the kinetic energy of the bombarding ions and the limited heat capacity of the sample (19). Although preferential sputtering of HzO or H from the Cd(OH)2 lattice could account for oxide formation, we feel that the dependence of these kinds of chemical changes on thermodynamic properties strongly suggests that the heating effect predominates to cause dehydration of the Cd(0H)z. Experiments are continuing in our laboratory to further elucidate this point for a wider variety of systems. From the 0 1s to Cd 3d5/2 intensity ratios, it is clear that some reduction to Cd metal is also occurring. This follows our previous experimental results which show that 400-eV Ar+ bombardment can cause reduction of oxide phases with a -AGfo less than 120 kcal/mole (19). The crystal structure of Cd(OH)2 is a layered structure (28) with each Cd atom surrounded by 6 hydroxide ions. Every OH- forms 3 bonds to Cd atoms in its layer. The crystal structure of CdO is a rock salt lattice. According to Low and Kamel (27), two overall steps are involved in the rearrangement from the Cd(OH)2 layered structure to the CdO rock salt structure. The initial step is dehydration,
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
533.0
33.0
5 30.0
530.0 BINDING
527.c
BINDING ENERGY (aV)
Figure 2. 0 1s electron spectra of (a) CdO, dehydrated at 350 O C under vacuum for 6 hours and transferred into the ESCA without exposure to atmosphere: (b) untreated commercial C d O (c) sample in (b) heated in situ at 200 O C for 1 hour; (d) sample in (c) heated at 300 O C for 1 hour: (e) sample in (d) at 25 O C after air exposure for 10 minutes
where H20 is lost leaving 0 2 - and OH- vacancies in the anion sublattice. The final step is rearrangement when the depleted layer lattice collapses to form the rock salt lattice. The fact that only OH- and 02-0 Is peaks are seen during dehydration supports the contention of Fahim and El-Salaam (20) that the increase and then decrease in B E T surface area as a function of increasing temperature of dehydration is due primarily to the vacant OH- position in the gross crystalline structure and that the microscopic crystal structure has only the Cd(0H)z and CdO lattice structures. The ESCA results do not support the theory of a "hydrooxide" presented by Huttig and Mytyzeh (29,30) and mentioned by Low and Kame1 (27), as an intermediate between the Cd(OH)2 and dehydrated CdO. The spectra of various forms of CdO are shown in Figure 2. The sample was obtained by heating a pellet of commercially available CdO in a vacuum furnace for six hours a t 350 "C. The vacuum furnace was operated in the inert atmosphere box to facilitate transfer of the purified CdO to the spectrometer without surface contamination. The untreated commercial CdO (Figure 2 b ) shows that the predominate contamination at higher binding energy is diminished by in situ heating a t 200 OC for one hour (Figure 2c). Further heating produces a less contaminated oxide (Figure 2d) which, when cooled and exposed to air for ten minutes, shows a reappearance of the contaminant (Figure 2 e ) . From comparison with the binding energy values in Table I, the contaminant could be either Cd(OH)2 or CdC03 or both, since our instrumental resolution cannot distinguish the two forms in the 0 Is region. The C Is spectrum of chemically prepared CdC03 reveals a C Is peak a t 289.1 eV
527.0
ENERGY (sV)
Figure 3. 0 I s electron spectra of (a) CdC03; (b) Cd02, spectra is skewed by sample charging: (c)CdOz exposed to 400 e V Ar+ ionbombardment (4 PA, 3 min.): (d)Cd02 heated in situ at 240 O C for 150 minutes
and an 0 ls/C peak area ratio of 1 O : l . Both of these values are consistent with the C I s binding energies and intensity ratios measured for Ag2C03 as reported in the next section. A comparison of 0 Is/C Is peak area ratios for several spectra of commercial CdO reveals that the carbonate contamination is from 30 to 40% and that the remainder is Cd(OH)2. Microanalysis of the commercial CdO reveals a bulk Cd(OH)2 concentration of 25% by weight with negligible carbon contamination. This discrepancy strongly suggests that the contamination is a surface species. Further understanding of the nature of these surface species can be gained from Ar+ stripping of a sample of commercial CdO. The reduction of this contaminant relative to bulk CdO is thought to be due to the sputtering process of the impinging ion in conjunction with the localized heating of the sample. This effect could account for both dehydration of Cd(OH)2 and decarbonation of CdC03. The rapid attenuation of the high binding energy 0 Is peak and the disappearance of the carbonate C Is peak with Ar+ etching, however, further supports their assignment as surface species. The 0 Is spectra of the chemically prepared CdC03 is shown in Figure 3a. As in the case of Cd(OH)2, thermal decomposition and differential charging of the insulator CdC03 are observed. Heating in situ produces a CdO species free from Cos2- or OH- with an 0 Is spectrum corresponding to Figure 2a. The 0 Is spectra of Cd02 (Figure 3 b ) as prepared after the procedure given in the Experimental section shows an 0 Is peak that is skewed, probably due to a slight CdO contamination. The use of in situ heating a t 240 "C for 150 minutes demonstrates the chemical transformation from peroxide to oxide (Figure 3c). Further evidence for the possible thermal effect of the Ar+ bombardment can be seen
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
e
2195
,
5 3 : O
532.0
""'4
Table 11. Intensities of Oxygen 1s Peaks from Oxygen-Exposed Evaporated Cadmium Surfaces Intensity'")
Dose
High
Low
B.E. peak
B . E . peak
1x Torr x 10 sec 1 1.2 0 1x Torr x 20 sec 20 2.2 1.4 3x Torr x 60 sec 180 2.3 2.3 6 x lom5Torr x 60 sec 3600 7.8 7.2 1.5 x 10" Torr x 120 sec 1.8 x l o 6 7.9 10.6 2 x lo-' T o r r x 330 sec 6.6 X lo6 7.6 12.5 2.8 x 10'' 12.4'b) 22.5 10' Torr x 28,800 sec Bulk CdO ... 35.1 Reported in counts/sec and evaluated by taking the total counts obtained at the peak maximum and dividing by the time that channel was "open" to counting. * A C 1s signal from the CO& binding energy region of 0.90 count/sec was noted. This would correspond to approximately 9.0 counts/sec in the 0 1s region and is undoubtably contributing to this peak.
* t z Y
1
i 533.0
530.0 B I N D I N G ENERGY
528.0
(ev)
Flgure 4. 0 1s electron spectra of (a)Cd metal evaporated in situ at
