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ACCELERATED ARTICLES
Mass Spectrometric Study of Rare Earth Oxide Equilibria in the Glow Discharge Yuan Mei and W.W.Harrison. Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
Glow discharge mass spectrometry has been used to study redox equilibria reactions of lanthanum and lanthanum oxide in an argon glow discharge. Introduction of the primary reagents of La and L a 0 is by sputter ejection from a cathodic sample. The plasma chemistry is greatly affected by oxidizing and reducing agents in the plasma, most prominent of which is residual water, shown here to reduce greatly the La/LaO ratio even at trace levels of water vapor. The injection of controlled amounts of water vapor was used to demonstrate this effect. Mixtures of A r and Ne permitted the study of atomization changes for Ag, Ti, and La samples. 180-enriched water was also used to follow oxidation processes in the plasma. Attempts were made to differentiate between oxygen reactants arising from sputtered oxide sample and those originating in the injected water.
INTRODUCTION In recent years, the glow discharge (GD) has received renewed interest as an analytical source for spectroscopic analysis of solid materials.' Both optical spectroscopy and mans spectrometry have been coupled successfullywith glow discharge sources for the analyses of conducting metals, alloys, and semiconductora.2~3Another group of solid samples, the nonconducting materials such as glasses, ceramics, and geologicah,is alsoattracting an increasing amount of research ~~
(1)Harrison, W.W.;Barehick, C. M.; Klingier, J. A.; Ratliff, P. H.; Mei, Y.Anal. Chem.1990,62,1965. (2) Jakuboweki, R.; Stuewer, D.; Toelg, G. Spectrochim. Acta 1991, &E, 156. (3) Mykytiuk, A.P.;Semeniuk, P.; Berman, S. Spectrochim. Acta Rev. 1990,13,1. 0003-2700/93/0385-3337$04.0010
using GD technique^.^*^ Two types of discharges have been used with nonconducting samples: direct current (dc) and radio frequency (r0.6,' When operated in the dc mode, the nonconductor sample must first be made conducting by mixing it with a metal matrix; the mixture is then pressed to form a cathode pellet. The use of rf GD sources is a relatively new approach for handling nonconducting solids,which does not require the matrix mixing step and thus simplifies the sample preparation procedure. Althoughtheoriginalresearch in thisarea was initiated in the 197Os,it was not until recently that the analytical utilites of rf GD sources were further explored. Complemented by their rf counterparts, dc glow discharge sources remain important for the analysis of nonconducting solids in both practical applications and fundamental research. The increased recognition that GDMS receives an an elemental analysis techniques can be attributed to several advantages, including the following: (1)ability for direct, rapid, and multielemental analysis of solids, (2) uniform response for most elements, (3) sub-ppb detection limit, (4) simplicity of mass spectra, and (5) ease of operation. Thew specialfeatures make GDMS avaluable technique for samples that exist in a complexsolid matrix and are difficult to diseolve. Many geological materials qualify for this sample category, which has essentially prompted the exploration of GDMS in the field of earth sciences. Among the most troublesome geological samples for spectroscopic analysis8 are the rare earth elements (REE). The difficulty is caused by the remarkably similar chemical and physical properties that the REE possess. Analytical techniques that are traditionally and currently used for REE (4) Winchester, M. R.; Hayes, S. M.; Marcue, R K. Spectrochim. Acta 1991,5B,615. (6)Ehrlich, G.; Stahlberg, U.; Hoffmann, V. 2.Spectrochim. Acta 1991,2,116. (6)ElAlfy, S.; Laqua, K.; Massmann, H.Z.Anal. Chem. 1979,263,l. (7) Duckworth, D. C.; Marcue, R. K. Anal. Chem. 1989,61,1879. (8)Javis, K.E.J. Anal. Atom. Spectrom. 1989,4,563. (9 1993 Amerlcen Chemkal Socbty
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analysis include atomic absorption spectroscopy: neutron activation analysis,l0X-ray fluorescence,ll spark source mass spectrometry,12and inductively coupled plasma (ICP) emisto sion spectroscopy and mass s p e ~ t r o m e t r y . ~Compared ~J~ solution-based methods, the simplified sample preparation of GDMS for direct solid analysis becomes an apparent advantage. Since the sputtering action of the glow discharge transforms the analyte sample directly from the solid state to atoms in the gas phase, sample dissolution and separation steps often required by other methods are eliminated. An early study of using GD sources for geological analysis was conducted by Marcus and Harrison,15in which a hollow cathode plume was used in combination with atomic emission spectroscopy to determine the chemical composition of a flint clay sample. In more recent years, Brenner et al. have reported using a GDAE system to determine both major and minor elements in avariety of solid geological materials.16 Research and application focusing on the use of GDMS for REE analysis have been active in several industrial and service laboratories,17 although due to proprietary reasons, publication in this area is limited. In a fundamental oriented study, King and co-workers have addressed the problem of molecular interferences when using GDMS to study several rare earth oxides.18 Molecular (monoxide in particular) interference is considered one of the major limitations involved with using GDMS for REE analysis. This type of interference arises from the chemical complexity of the plasma and is directly associated with the oxidic nature of the sample. One complexity arises from the cosputtering of monoxide molecules of the analyte along with the REE atoms. Insufficient dissociation of the tenacious REE-oxygen bonds during the sputtering step is possibly the main cause for the ejection of monoxide m o l e ~ u l e s . ~Furthermore, 9~~~ monoxide molecules may also form in the discharge gas phase as a result of combination reactions between the REE and gaseous impurities. These gaseous impurities are pressed within the cathode during disk preparation and can be released into the plasma by sputtering. Studies have shown that, even when present at low concentrations, gaseous contaminants such as water may affect severely the plasma chemistry of the analyte.2l For a strongly bound oxide sample like the rare earths, the effects that gaseous impurities have on the analyte atom may be shown by the generalized 'redox- equilibrium depicted in eq 1, where M and MO stand for atomic and MO+M+O
(1)
monoxide species of the analyte, respectively. Generally speaking, for elemental mass spectrometric analysis, a high atomic (M) ion concentration is desirable in order to achieve good analytical sensitivity, which necessitates the minimization of MO molecules. According to eq 1, the oxygen concentration in the system is a crucial factor to the (9) Ooghe, W.; Verbeek, F. Anal. Chim. Acta 1974,73,87. (10)Smith, A. D.; Gillis, K. M.; Ludden, J. N. Chem. GeoZ. 1990,81, 17. (11)Sholkovitz, E. R. Chem. Geol. 1990,88,333. (12)MeLennan, S. M.; Taylor, S. R. Chem. Geol. 1980,29,333. (13)Rathi, M. S.;Khanna, P. P.; Mukherjee, P. K. Talanta 1991,3, 329. (14)Date, A. R. Spectrochim. Acta Reu. 1991,14,3. (15)Marcus, R. K.;Harrison, W. W. Anal. Chem. 1987,59,2369. Anal. Chem.Spectrom. (16)Brenner,I.B.;Laqua,K.;Dvorachek,M.J. 1987,2,623. (17)Application notes, VG Isotopes, Manchester, England. (18)King, F. L.; McCormack, A. L.; Harrison, W. W. J.Anal. Atom. Spectrom. 1988,3,883. (19)Wurz,P.;Husinky, W.; Betz, G.; Appl. Phys. 1991,52A,213. (20)Coburn, J. W.;Taglauer, E.; Kay, E. J p n . J. Appl. Phys. 1974, Suppl. 2,Pt 1, 501. (21)Ratliff, P.H.; Harrison, W. W., paper in preparation.
equilibration between M and MO. An equilibrium involving one of the most tenacious oxides in the REE group, lanthanum oxide (La2Oa), is investigated in this work. In constructing the M-MO redox equilibrium in the glow discharge, the first step is the ejection of neutral M atoms and MO molecules into the gas phase through cathodic sputtering. Following the ionization of these neutral species, the ion signals of M+ and MO+ are detected by the mass spectrometer. The fraction of atomic ion signal to the total analyte ion signals, i.e., M+/(M++ MO+),can be used as one indication of how complete the monoxide molecules are dissociatedto atoms. For different oxides,this ratio decreases as the M-0 bond energy increases.20 Since the REE form some of the strongest oxide bonds (e.g., L a 4 8.3 eV), dissociation of the La-0 bond becomes one of the most important and difficult steps in the GD process. Previous studies have shown that one way of reducing monoxide interference is using getter reagents as the conducting sample matrix.z2 Getter reagents are highly reducing metals that readily react with speciescontaining oxygen, nitrogen, carbon, and hydrogen. It was proposed that the successfulconversion of La0 to La achieved by using getter reagents as sample matrices (with La+/(La++ Lao+)reaching 98%)was a result of sputtered getter atoms reacting with oxygen and depleting it from the plasma. Relating this explanation to eq 1,it would be equivalent to shifting the La-La0 equilibrium to the formation of La by removing 0 from the redox system. To further our understanding of the redox processes involving La203 in the glow discharge, a different approach is taken to affect this equilibrium. An argon plasma, which has already equilibrated in terms of obtaining the largest La population, is first created. The equilibrium is then disturbed by introducing into the steady-state plasma reagent gases having different redox properties. Consequently, the LaLa0 system is expected to undergo reequilibration, a process from which information about the plasma chemistry of the oxide may be obtained. The experiments should be most informative when a spectrum of gases with distinctlydifferent chemical nature is used as added 'reagents". The selected gases include the chemically inert (neon),reducing (hydrogen), and oxidizing (water vapor).
EXPERIMENTAL SECTION The mass spectrometer system used in this study has been described in a previous publication.B The analyte La203 (RG, Fisher Scientific Co.) was mixed, 10% by weight, separatelywith three matrices: tantalum (SG, Spex Industries, Inc.), titanium (99.9%, Aldrich Chemical Co.), and silver (99.999%,GallardSchlesinger Chemical Mfg. Corp.). The mixtures were then pressed to form disk cathodes. A pure AI discharge was run initially to clean the sample surface and to maximize the La+/ Lao+ratio before adding any one of the following reagent gases: Ne (99.999%,Spectra Gases Inc.), HZ(99.95%,Liquid Air Co.), HzIsO (distilled),and HZ'~O(97 atom % , Sigma Chemical Co.). The total discharge pressure was kept at 0.95 Torr; the partial pressure of the added gases was set in the range of 0-0.95 Torr for Ne and Hz,and 0-0.2 Torr for the vapor of both H P 0 and Hzl*O, while the remaining partial pressure was made up by Ar gas. The discharge was reequilibrated in pure AI to ita best reproducible stage (in terms of obtaining the same La+/LaO+ ratio) before each reagent gas addition at a new pressure setting. Signal intensities for both La+ and Lao+ were monitored prior to and after the addition of the reagent gases. In the case of Ne addition, atomic absorption and weight loss studies were also carried out to monitor changes in the sputter yield of the matrix metal. The discharge was typically run at a constant lo00 V. The discharge current varied as the mixing ratio of the discharge gases was changed. (22)Mei, Y.;Harrison, W. W. Spectrochim. Acta 1991,&B, 175. (23)King, F.L.; Harrison, W. W. Mass Spectrom. Rev. 1990,9,285.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
SSSO
RESULTS AND DISCUSSION Experiments presented in this paper were focused on the chemical manipulation of the redox equilibrium depicted by eq 1. The main variable that was changed to cause shifting of the oxide equilibrium was the oxygen concentration (contributed by various oxygen-containing species) in the plasma. Such changes were introduced in two ways: (1)by reducing the amount of getter reagent in the plasma and hence increasing the oxygen concentration in the redox system and (2) by directly adding oxygen provided by water vapor into the discharge gas. The induced effects on the La-La0 equilibrium were studied as a function of amounts of getter and water in the discharge. Effect of Sputtered Getter Content on La-La0 Equilibrium. Addition of Neon. In these experiments, Ne was introduced into the discharge to cause concentration changes of the sputtered materials in the plasma. Because of ita inert nature, Ne added to an Ar discharge should not participate directly in the redox reactions with the oxide analyte, Lao. However, because Ne is a less efficient sputtering gas than Ar, replacing part of the Ar with the lighter Ne is expected to decrease the sputter yield of all constituents in the sample cathode," including the matrix metal (Agor Ti), 0 atoms, La atoms, and L a 0 molecules. The degree of sputter yield decrease may not be the same between La and Lao, which would lead to changes in the La/LaO ratio when Ne is added. But if this is the only source for any possible La/LaO ratio change upon adding Ne, the change in the La/LaO ratio should be similar whether Ag or Ti is used as the matrix. However, the matrix of the cathode reacts with and hence helps scavenge oxygen-containing species in the plasma (as proposed to be the case for getter reagents),and then a decrease in the sputter yield of the cathode may cause leea efficient removal of oxygen from the discharge. Oxygen arising from sample sputtering is considered to be small compared to the contributions of water vapor desorbed from chamber walls and the compacted sample. Thus, reducing the getter population is equivalent to increasingindirectly the oxygen concentration in eq 1,and the La-La0 equilibriumwill shift accordingly. In this process, the amount of sputtered matrix material in the gas phase is the main variable that influences the La-La0 equilibrium. If the matrix metal is not involved in the La-La0 equilibrium, then reducing ita concentration should not affect the equilibrium significantly. If the matrix metal does participate in the La-La0 equilibrium,such as proposed for getter matrices, then the La-La0 equilibrium should be affected by the concentration change of the matrix metal. By comparing results obtained from using both getter and nongetter matrices, further understanding of the getter reagents' role in the glow discharge plasma chemistry may be attained. Although neon has been shown to have smaller sputter yields than Ar in the glow discharge due to ita lighter atomic weight, most comparative sputter yield studies done between Ar and Ne have involved individual sputter yield measurementa made separately for both gases.26 Sputter yields for mixed gases, however, were not found in the literature. In a constant-voltage discharge, the discharge current cannot be held constant when Ar and Ne are mixed at various ratios, because Ne and Ar gases have different ionization efficiencies and do not yield the same current values under a fixed voltage. As a result of the changing current, sputter efficiencies of gases mixed in different ratios cannot be directly compared. Since our goal was not to compare absolute sputter efficiencies of the two gases, but to create a change in the sputtered species concentrations and then relate such a change to the oxide (24) Roeenberg, D.;Weber, G . K.J . Appl. Phys. 1962,33,1&92. (25) Wagatsuma, K.;Hirokawa, K.J. Anal. Atom. Spectrom. 1989,4, 525.
N
E
0.20
t
8 0.00 0.0 0.2 0.4 0.6 OB 1 .o Ne Partial Pressure (Torr)
on the sputtered weight loss of Ag. equilibrium, a constant-discharge current was not necessary. To monitor the sputter yield change in these experiments, both weight loss and atomic absorption measurementa were carried out. Measuring weight loss after sputtering the sample at a constant current for a given period of time is the most common way of evaluating net sputter yield.27 However, metals that are highly reactive toward oxygen (e.g., getter reagents) are not suitable for weight loss studies, because severe oxidation reactions will occur once the sample is removed from the vacuum chamber into the air, causingweighing errors. Weight loss measurements were then carried out on a Ag sample instead of the highly reactive Ti; the goal was to examine whether less sample material was sputtered when Ar was replaced by Ne. Figure 1 reflects the weight loss of Ag, normalized to that obtained by using pure Ar, vs the partial pressure of Ne in a series of Ne-Ar mixtures. There is an apparent decreasing trend in the sample weight loss with the increasing portion of Ne in the discharge gas. The net sputter weight loss of Ag decreased about 60% when the gae was changed from pure Ar to pure Ne. Because sputter weight loss is affected by both the sputtering efficiency and the redepositionrata of the sputtered material, a reduced weight loss is not necessarily completely attributable to reduced sputtering by Ne. The weight loas method only measures the net sputter yield over a time integral, but does not provide information on the sputtering efficiency at any particular time. Therefore, to conclude that there was an actual decrease in the sputtered species population in the gas phase, it was necessary that the weight loss studies be complemented by atomic absorption methods, which measure directly the sputtered neutral population in the gas phase. In addition, since the weight loss methods were not suitable for reactive samples such as Ti, atomic absorption measurements carried out with a Ti cathode can provide a means of monitoring the effect of adding Ne on the atomic population of this getter reagent in the glow diecharge. The absorbance of the Ti0 was measured at five Ne/& mixing ratios and normalized to the absorbance of the Tio obtained in the pure Ar discharge. The normalized absorbance is plotted against the corresponding Ne partial pressure in Figure 2, which showsa decreasingtrend of the TiOabsorbance in general agreement with the sputter weight loss data of Ag shown in Figure 1. Results from both experiments confined that the concentration of the sputtered species in the plasma was reduced when Ar was replaced by Ne. Whether this effect would cause the La-La0 equilibrium to shift was next examined. Figure 1. Effect of Ne
(26) Boumans, P.W . J. M . Anal. Chem. 1972,44, 1219. (27) Stern, E.;Caswell, H.L. Vuc. Sci. Technol. 1966,4, 128.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
1.oo
0.80 0.60 0.40
L
\\
1
= 0.000.0
0.2
0.4 0.6 0.8 Ne Partial Pressure (Torr)
0
11.20
0.00
0.0
0.2
1.o
0.4
0.6
0.8
1.o
H2 Partial Pressure (torr)
Effect of HP partial pressure on the La+/LaO+ ratlo for a La20s-Ag sample. Figure 4.
Effect of Ne partial pressure on the absorbance of
Figure 2.
0.20
TIo.
r
[
Ta matrix
+,
+
La203in AQ
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.-+0
a
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i
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z
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'
'
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'
'
oa
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'
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'
'
oa
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Ne Partial Pressure (torr) Comparison of effect of Ne an the La+/LaO+ ratio between Ag and Ti matrices. Figure 3.
If adding Ne to Ar does change the ion signals of La+ and Lao+, then such effects may be brought about in two ways: (1)'direct effects" including preferential sputtering and/or biased ionization between La and La0 and (2) 'indirect effects" manifested in changes of two ion signals as a result of chemically shifting the La-La0 equilibrium. Indirect effects can be initiated by the concentration change of oxygen in the plasma and, hence, by the concentration change of species that react with oxygen such as getters. The degree of neon's direct effects on the La-La0 equilibrium needed to be determined first before any indirect effects could be sized. To examine experimentally the direct effects only, causes for indirect effects must be ruled out by using a less oxygen-reactive matrix (e.g., Ag). Ne addition experiments were conducted using a Ag cathode containing 10% Laz03. Following the optimization of the La+/LaO+ratio in Ar, Ne was bled into the system to replace an equivalent (by pressure) amount of Ar. For each Ne/Ar mixing ratio, the signal ratio La+/LaO+ was normalized to the value obtained in pure Ar (Figure 3). Data are plotted as the normalized La+/LaO+ratio vs the corresponding Ne partial pressure. For the Ag matrix, the La+/LaO+ ratio showed a small decrease before 75% of the Argas was replaced by Ne, and a larger drop upon reaching 100%Ne. The same experiments were then conducted with a Ti matrix, which yielded a more pronounced reduction in the La+/LaO+ratio over the same range of pressure change, as shown in Figure 3.
The fact that up to a point where a larger portion of Ar was replaced by Ne, the La+/LaO+ratio remained fairly stable with the Ag matrix suggested that (1)the ratio of sputtered neutral La/LaO was not significantly affected by replacing
0.1 0.00
0.03
0.06 0.09 0.12 0.15 0.18 Hh60 Partial Pressure (Torr) Figure 5. Comparison of water vapor's effect on the La'60+lLa+ ratlo
between Ta and Ay matrices. Ar with Ne until this point and (2) any preferential ionization that might be caused by the more energetic Ne metastable atoms (overAr metastables) was negligible. Overall,addition of Ne did not seem to have a significant direct effect on the oxide signals until the Ar concentration was below 20% of the total gas pressure. Indirect effects, as demonstrated by the more widely changing La+/LaO+ratio with the Ti matrix, were more pronounced. Furthermore, by comparing curve 2 in Figure 3 with Figure 2, it is apparent that the decreasing La+/LaO+ratio was in close agreement with that of the Tio absorbance. This suggests that reducing the supply of Ti into the gas phase (as evidenced by Figure 2) was essentially responsible for shifting the La-La0 equilibrium toward the formation of Lao, shown as the declining La+/LaO+ratio in Figure 3. Addition of Hydrogen. The second reagent added, hydrogen, may also significantly reduce the sputter yield from that using Ar due to the large amount of light-mass ions, primarily H+and H3+,carrying the discharge current.*' But unlike Ne', hydrogen possesses a more active chemical nature. The reducing nature of hydrogen has been used in ICP sources to help eliminate molecular interferences by dissociating strongly bound oxides.28 Of interest in our experiment was whether hydrogen could induce favorable reducing reactions in the glow discharge, where the thermal energy is much lower than in an ICP source. Hydrogen addition experiments were carried out on the LazOS-Ti sample, and the normalized La+/LaO+ratio was plotted against the partial pressure of Hz in Figure 4. The La+/LaO+ratio showed a large decrease as Ar was partially (28) Powell, M. J.; Boomer, M. J.; McVikars, R.J. Anal. Chem. 1986, 58, 2867.
ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
replaced by Hz. This decrease indicates that not only was there no net reducing effect on La0 when hydrogen was added into the glow discharge (as in the case of ICP), but also that the relative proportion of L a 0 increased. The cause for this phenomenon may be a combination of changes in the sputtering and ionization mechanisms introduced by the addition of hydrogen. While monitoring the neutral Ti population by atomic absorption in the same manner as described in the Ne addition experiments, we observed an even more dramatic decrease in the absorbance of Tio when even a minimum amount of Hz was added. Thus, the decreasing trend of the La+/LaO+ratio as Hz was added into Ar may be explained in the same fashion as with the Ne addition; that is, a less reducing environment was created due to the decreased concentration of the getter reagents sputtered into the discharge. Moreover, the decrease in the La+/LaO+ratio with the use of HZwas greater than when Ne was used. This may be attributed to the even smaller sputter efficiency of H+ than Ne+, resulting in a larger decrease in the Ti concentration in the gas phase. Effect of Enhancing Water Content on La-La0 Equilibria. Water, when used as a plasma reagent, can provide oxygen directly to the right side of eq 1and is expected to shift the La-La0 equilibrium toward the formation of Lao. Factors that may be attributed to the shift include the following: (1)less dissociationof the sputtered La0 molecules in an oxidizingenvironmentand (2) formation of La0 through free atomic La reacting with added water and its dissociated products, either on the sample surface, in the gas phase, or both. In an attempt to c o n f i i and differentiate between these two possibilities, isotopically labeled water (HzW) was used as an added gas. Oxidation reactions between free La and added H P O (including either on the sample surface or in the gas phase) could be confirmed if the new species La180, which did not exist in the sample, was detected by the mass spectrometer. In addition, a better understanding of getter reagents' role in the glow discharge chemistry may also be gained through these experiments. Addition of Watel-HP0. These experiments were designed to disturb the La-La0 equilibrium by providing additional oxygen through water. Principal reactions involving water molecules in the Ar glow discharge have been studied by LindingeP and are demonstrated by the following equations:
+ ArH+ + H,O = H30++ Ar H20++ H,O = H30++ OH ~ r ++H,O = H,O+ + ~r H30++ e = H,O + H ~ r ++H,O = A ~ H + OH
(2)
(3) (4)
(5) (6)
The oxygen-containingfragments from water dissociation may be capable of oxidizing La atoms to form La0 molecules in the discharge. Since the interest here was to promote La0 production rather than dissociation, the signal ratio of Lao+/ La+ (rather than La+/LaO+)was used for data discussion. The signal ratio of LaO+/La+obtained from both Ag and Ta matrices is plotted as a function of the partial pressure of HzO in Figure 5. Because water has been reported to quench the Ar metastable ionization of sputtered species,3Oan overall decrease in all ion signale of the sputtered species was expected as a result of adding water. The additional oxygen provided to the discharge from the added water (and/or water (29) Lindinger, W. Phys. Reu. 1973, 7, 328. (30) Velazco, J. C.; Knolts, J. H.; Setaer, J. W.J. Chem. Phy8. 1978, 69, 4357.
8841
mh Flguro 6. Production of Lal80In the gbw discharge by adding H2180 vapor: (a) mass spectrum obtained before H2"0 addltbn; (b) mess spectrum obtained after H2'*0 addbn.
lo'
--
"1 2lo
lo.'
0.00
"
"
0.03
0.06
"
"
OD9 Ma0 Partial Pressure (Torr)
0.12
"
0.11
7. Ion anak of La+, La%+, and La%+ as functions of the H2% partlel pressure.
fragments) may also react with La atoms to form La0 molecules. La0 thus produced may have compensated for some of the loes in the Lao+signal caused by reduced Penning ionization. Hence, the decrease in the absolute Lao+ signal was not as large as that in the La+ signal, resulting in an increased LaO+/La+ratio with added HzO partial pressure. There appeared to be a threshold in the H20 partial pressure between 0.07 and 0.15 Torr where the LaO+/La+ ratio increased drastically. This phenomenon was observed for LaO+/La+ratios obtained from both Ta and Ag discharges. The change in the LaO+/La+ratio was so abrupt that the appearance of the plots almost resembles that of a titration curve, which was more pronounced with the Ta matrix than with Ag. Although the cause of such curve shapes is not completely understood, some speculation may be made in an attempt to explain this phenomenon. Near the 'equivalence point" in Figure 5, the slope of the curves seems to be dependent on the matrix metal (greater slope for Ta than for Ag). This suggests that the LaO+/La+ratio was possibly influenced by the matrix metal and the added water vapor. Similar results have been obtained by Brundy and Wittmack when studying the effecta of oxygen pressure on the sputter yields of several transition metals.31 They observed abrupt decreases in the sputter yields of reactive metals (e.g., Ti) once the 02 partial pressure exceeded a threshold value. One explanation for this pseudotitration-type transition is that once the water (31) Brundy, M. M.; Wittmack, K. NucI. Z R P ~ ~ U MMethod8 . 1991,96, 42.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 23, DECEMBER 1, 1993
concentration exceeded its value at the equivalence point of the redox equilibria in the plasma, the cathode surface was quickly covered with excess oxygen, oxidizing the cathode metal as well as reducing ita sputter yield. As a result, the reducing environment created by sputtering Ta could no longer be maintained, and the La-la0 system was quickly dominated by the formation of La0 molecules. It cannot be excluded that similar redox equilibria may also exist in the gas phase. The oxidation of the sputtered T a atoms in the gas phase may be the main cause for the loss of the reducing plasma, even if the cathode surface was still metallic in nature at this point. With the reactive gas being introduced into the plasma at a constant pressure, it is difficult to distinguish between reactions happening on the cathode surface or in the gas phase. Chemical interactions that occur primarily in the gas phase will be further addressed in a separate paper,3Zwhile a preliminary examination is provided by the following experiment. Addition of Wuter-H2W. The oxygen atom released from regular water (H2160)has the same dominant isotope l60as the oxygen atoms originatingin the La203 sample. Therefore, the origin of the increased Lao+ signal upon adding H2160 could not be resolved from the following possible sources: (1) recombination reactions between La and added water or (2) less dissociation of originally sputtered La0 molecules. To distinguish between these two processes and to provide further evidence of gas-phase reactions between sputtered La and added water, 180-labeledwater (Hzl8O)was used as a plasma “reagent” to replace regular water. Upon the addition of Hzl8O,a new species at m/z157, Lal80+,was observed in the spectrum shown in Figure 6b. Not originating from the oxide sample, the new species was considered a reaction product between La and added H2180. Introduction of H2180into the discharge caused the analyte atom La to redistribute among ita three forms: La, Lal60, and La180. When the concentration of H2180 was changed, the equilibria among the three species were expected to shift accordingly,so that concentrations of La+,La160+,and La180+ would also change. Figure 7 demonstrates the signal profiles of the three lanthanum species as the amount of H P 0 was increased. Starting from zero H2180 concentration, La180 became the dominant species as more H2l8Owas bled in the discharge. This suggests that a competitive reaction mechanism was responsible for the redistribution of La in its two oxide forms. Similar to H2l60+addition, the pseudotitration phenomenon was also observed in the plot of the La180+/La+ ratio vs H2180 partial pressure in Figure 8. The highly reducing discharge created by T a acted as an “oxygen buffer” until its reducing capacity was saturated by added water, and then the drastic increase in the La180+/La+ratio occurred. Whether the La180 molecules were formed primarily by (32) Mei, Y.; Harrison, W. W., paper in preparation.
‘m 19.0 240 %14O
1
f I
in Ta matrix
i ? [ i r
0
.-0
Y 0.00
0.03
0.06 0.09 0.12 Hh*O Partial Pressure (Torr)
0.15
0.18
Figure 8. Lal80+/La+ratio as a functlon of the H2180partial pressure wtth both La20s-Ag and La203-Ta samples.
oxidation of the cathode surface or in the gas phase is not completely understood, but results from these studies do indicate that an oxidation reaction between La and water does occur in the glow discharge as long as there are sufficient reactants.
CONCLUSIONS The reversibility of the La-La0 redox equilibrium in the glow discharge has been studied by chemically manipulating the plasma chemistry through various gas additions. A discharge that is “reducing” in nature enhances the atomic population of the rare earth oxide anal*. The reducing effect of getter reagents in the glow discharge can be weakened when using less sputter-efficient discharge gases. This will result in a decrease in the atomic population of the oxide analyte relative to ita monoxide population. The use of isotopicallylabeled water as a plasma reagent has shown that oxidation reactions between La atoms and oxygen atoms not originating in the sample itself are possible in the glow discharge. In this study, the gas mixing and introduction processes were carried out in the steady-state mode. This determined that the added gas not only would act as reagents for gas-phase reactions, but also would take part in the sputtering process of the discharge.
ACKNOWLEDGMENT We are grateful for the support of our research by the Department of Energy, Division of Chemical Sciences.
RECEIVED for review August 3, 1993. Accepted September 24, 1993.” @
Abstract published in Advance ACS Abstracts, October 15, 1993.
