SURFACE AND INTERFACE ANALYSIS, VOL. 25, 161È166 (1997)

Changes in the Chemical State and Composition ofthe Clean Surface of and due toK

2CrO

4K

2Cr

2O

7Air Exposure and Argon Ion Bombardment

Shigeru Suzuki,*1 Masaoki Oku2 and Yoshio Waseda11 Institute for Advanced Materials Processing, Tohoku University 2-1-1 Katahira, Sendai 980-77, Japan2 Institute for Materials Research, Tohoku University, Sendai, Japan

X-ray photoelectron spectra have been used for characterizing changes in the chemical state and composition of theclean surface of single (potassium chromate) and (potassium dichromate) crystals with Cr(VI)K

2CrO

4K

2Cr

2O

7due to air exposure and argon ion bombardment. Their clean surface was prepared by cleaving these crystals inUHV, and XPS spectra from the surface were successfully obtained only by optimizing conditions of the neutral-izer because of non-conductivity.

The chemical state of constituent elements in the surface of and was not signiÐcantly a†ectedK2CrO

4K

2Cr

2O

7by air exposure, although the surface concentration was changed. On the other hand, argon ion bombardmentconsiderably induced reduction of chromium ions, Cr(VI) to Cr(III), in and while the chemicalK

2CrO

4K

2Cr

2O

7,

state of potassium and oxygen is almost unchanged by the bombardment.It was also found that the preferential sputtering of potassium in and takes place during theK

2CrO

4K

2Cr

2O

7argon ion bombardment, and the degree of the preferential sputtering in is about twice as high as that inK2Cr

2O

7From the correlation between the variation in the chemical state of chromium and the preferentialK2CrO

4.

sputtering of potassium, the reduction behavior of chromium ions by the ion bombardment is considered mainlyattributed to the preferential sputtering of potassium from the surface of and crystals. 1997K

2CrO

4K

2Cr

2O

7(

by John Wiley & Sons, Ltd.

Surf. Interface Anal. 25, 161È166 (1997)No. of Figures : 10 No. of Tables : 0 No. of Refs : 20

KEYWORDS: x-ray photoelectron spectroscopy ; ion bombardment ; preferential sputtering ; chemical shift ; potassium;chromium

INTRODUCTION

X-ray photoelectron spectroscopy (XPS) is one of themost powerful methods to investigate the chemical stateof constituent elements in the solid surface. In particu-lar, this method has often been employed for studyingthe valence number of cations in oxides. Since XPS isalso one of the surface analysis methods, attention mustbe paid to preparation of the specimen surface ; forinstance, the surface of metals and alloys may be easilyoxidized by exposure to air. It is also known that argonion bombardment sometimes induces the reduction ofoxides, such as titanium oxide and nickel oxide,1h7 andeven x-ray irradiation can induce the reduction ofoxides.8h11

Characterization of inorganic compounds containingCr(VI) by XPS is of great interest in relation to environ-mental problems. Potassium chromate and(K2CrO4)potassium dichromate contain typical com-(K2Cr2O7)pounds with Cr(VI), and the peak position of Cr 2pXPS spectra has been reported by a fewresearchers.12h14 However, the results were obtained bymeasuring their reagents simply, without careful atten-tion to surface conditions such as contamination.Therefore, bulk specimens produced by sufficiently con-

* Correspondence to : S. Suzuki.

sidering such points should be used to investigate theiroriginal XPS spectra and the e†ect of surface prep-aration on the XPS spectra.

The main purpose of this work is to provide informa-tion for the XPS spectra obtained from the clean surfaceof single crystals of and and theK2CrO4 K2Cr2O7inÑuence of air exposure and argon ion bombardmenton the XPS spectra. Moreover, the mechanism ofchanges of their surface due to these treatments will bediscussed from the relationship between the chemicalstate and surface concentration.

EXPERIMENTAL

Specimens

Single crystals of and were grownK2CrO4 K2Cr2O7from their aqueous solution by drying them slowly atroom temperature. They were cut to bars of D2 mmsquare and 6 mm length. The bar specimen was insertedinto a copper specimen holder and introduced into anultrahigh vacuum (UHV) of D5 ] 10~10 Torr. It wascleaved in situ in a fracture chamber and subsequentlymoved to the XPS spectrometer. Figure 1 demonstratesan example of a crystal cleaved in UHV.K2CrO4Although the cleaved surface with some roughness mayinÑuence Ðne structures of XPS spectra, we can accept

CCC 0142È2421/97/030161È06 $17.50 Received 16 July 1996( 1997 by John Wiley & Sons, Ltd. Accepted 13 November 1996

162 S. SUZUKI, M. OKU AND Y. WASEDA

Figure 1. Photograph of a crystal cleaved in UHV.K2CrO

4

systematic changes in XPS spectra by treatment. TheXPS spectra from only the cleaved surface wereobtained for discussing the chemical state and concen-tration of the surface, while for energy calibration thespectra from the copper holder together with the speci-men were recorded by moving the analysis position.

Measurements

X-ray photoelectron spectroscopy was carried out byusing a PHI Model 5600 spectrometer with monochro-mated Al Ka radiation. Since the present specimens arenon-conductive, conditions for a neutralizer were opti-mized to obtain XPS spectra. The Cr 2p, K 2p and O 1sXPS spectra were mainly measured, and the energypositions of the spectra were calibrated using Cu 2p at932.7 eV and Cu 3p at 75.1 eV for the copper holder.

The e†ect of air exposure on XPS spectra was investi-gated by exposing the specimens to air for 300 s atroom temperature. Ion bombardment was performed byirradiating argon ions of 1 kV and 0.7 A m~2 using anattached ion gun.

RESULTS AND DISCUSSION

Changes in XPS spectra in a crystal by airK2CrO

4exposure and argon ion bombardment

Figure 2 shows Cr 2p XPS spectra from a K2CrO4crystal cleaved in UHV, subsequently exposed to airand further bombarded by argon ions for 60, 120, 240and 600 s. The Cr peak is observed at 580.0 ^ 0.22p3@2eV in the as-cleaved specimen, which is not signiÐcantlychanged by the air exposure. This value is comparableto a peak position in a reagent specimen reported in aprevious work,13 in which details for the method ofsurface preparation are not described. A new peak cor-responding to Cr(III)16,17 is found to appear and itsintensity increases with argon ion bombardment, indi-cating that the bombardment induces reduction ofCr(VI) to Cr(III) in the surface.

In Fig. 3 the K 2p XPS spectra from a K2CrO4crystal cleaved in UHV, exposed to air and bombardedfor 60, 120, 240 and 600 s are given. The K peak2p3@2at 292.7^ 0.2 eV is likely to be independent of air expo-

Figure 2. Chromium 2p XPS spectra from (a) cleaved inK2CrO

4:

UHV; (b) exposed to air ; bombarded by argon ions for 60 s (c),120 s (d), 240 s (e) and 600 s (f).

sure and ion bombardment. The shapes of the spectraare somewhat a†ected by the ion bombardment, whichmay result from surface damage formed by argon ions.As shown in Fig. 3(b), the C 1s peak is also observed at285.0^ 0.2 eV in the specimen exposed to air, which ismainly due to adsorption of hydrocarbon. The C 1speak readily disappeared on ion bombardment.

Figure 4 shows the O 1s XPS spectra from a K2CrO4crystal cleaved in UHV, exposed to air and bombardedfor 60, 120, 240 and 600 s. The main peak is located at530.3^ 0.2 eV, irrespective of the air exposure andargon ion bombardment. This suggests that the mainchemical state of oxygen in its surface was almostunchanged by these treatments, but ion bombardmentappears to enhance the peak broadening to form otherstates that may be attributed to atomic defects formedby ion bombardment.

The XPS spectra in the close vicinity of the valenceband for a crystal cleaved in UHV, exposed toK2CrO4air and bombarded for 60, 120, 240 and 600 s are givenin Fig. 5. It is also noted that the lower binding energypeak increases in the lower valence band peak with ionbombardment, indicating that the surface is reduced toa smaller bandgap species. The XPS spectra demon-strate peaks of O 2s at D20 eV, K 3s at 31 eV, K 3p at16 eV and Cr 3p in the range 52È56 eV. It is found that

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 161È166 (1997)

CHEMICAL STATE CHANGES IN K2CrO4 AND K2Cr2O7 163

Figure 3. Potassium 2p XPS spectra from (a) cleaved inK2CrO

4:

UHV; (b) exposed to air ; bombarded by argon ions for 60 s (c),120 s (d), 240 s (e) and 600 s (f).

the Cr 3p peak from Cr(III) appears at D52 eV, next tothe peak from Cr(VI) at D56 eV, by applying argon ionbombardment. This also indicates the reduction ofCr(VI) to Cr(III). On the other hand, the peak positionsin K 3s, K 3p and O 2s XPS spectra are almostunchanged. These characteristic features are consistentwith the results as given in Figs 3 and 4, and in additiona change in the structure of the valence band of \5 eVis recognized.

Changes in XPS spectra in a crystal by airK2Cr

2O

7exposure and argon ion bombardment

The Cr 2p XPS spectra from a crystal cleavedK2Cr2O7in UHV, exposed to air and bombarded by argon ionsfor 60, 120, 240 and 600 s are shown Fig. 6. The Cr

peak at 580.0^ 0.2 eV in this as-cleaved specimen2p3@2is in good agreement with the peak energy reported pre-viously,15 and almost the same as that for K2CrO4shown in Fig. 2. However, an additional shoulder isfound in the low-binding-energy side which may orig-inate from some imperfection remaining in this surface.The e†ects of air exposure and argon ion bombardmenton the Cr 2p, K 2p and O 1s XPS spectra in K2Cr2O7were fundamentally similar to those for ItK2CrO4 .

Figure 4. Oxygen 1s XPS spectra from (a) cleaved inK2CrO

4:

UHV; (b) exposed to air ; bombarded by argon ions for 60 s (c),120 s (d), 240 s (e) and 600 s (f).

should be noted from comparison of Figs 6 and 2 thatthe reduction rate of Cr(VI) to Cr(VIII) in byK2Cr2O7argon bombardment is considerably higher than thatfor K2CrO4 .

The XPS spectra in the low-binding-energy rangefrom cleaved in UHV, exposed to air andK2Cr2O7bombarded for 60, 120, 240 and 600 s are given in Fig.7. The 3p peak of Cr(III) at D52 eV signiÐcantlyincreases and the shape of the valence band changeswith the argon ion bombardment, while the positions inthe K 3s, K 3p and O 2s peaks are little a†ected. Theseresults straightforwardly correspond to the fact that thereduction rate of Cr(VI) in by the bombard-K2Cr2O7ment is high.

Changes in the surface compositions of andK2CrO

4by air exposure and ion bombardmentK2Cr

2O

7

In order to consider the mechanism of reduction behav-ior in the surface of and crystals, theK2CrO4 K2Cr2O7variations of the surface concentrations of chromium,potassium and oxygen induced by air exposure andargon ion bombardment are shown in Figs 8 and 9respectively. The concentration is estimated from theintensities of the XPS spectra coupled with their sensi-tivity factors. In this plot, carbon is excluded to see the

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 161È166 (1997)

164 S. SUZUKI, M. OKU AND Y. WASEDA

Figure 5. X-ray photoelectron spectra in the low-binding-energyrange from (a) cleaved in UHV; (b) exposed to air ; bom-K

2CrO

4:

barded by argon ions for 60 s (c), 120 s (d), 240 s (e) and 600 s(f).

relative changes in the surface concentration of the con-stituent elements. The oxygen concentration slightlyincreases on air exposure, while the chromium and pot-assium concentrations decrease. This may arise from theoxygen adsorption on the surface during air exposure.It is found that the chromium concentration increaseswith increasing argon ion bombardment time, whereasthe potassium concentration clearly decreases and theoxygen concentration also slightly decreases. This indi-cates that potassium is preferentially sputtered from thesurface and chromium remains in the surface. As alsoshown in Fig. 9, the fundamental e†ects of air exposureand argon ion bombardment on the surface concentra-tion in are analogous to the case forK2Cr2O7 K2CrO4 .However, it is noted that an increase of the chromiumconcentration and a decrease of potassium concentra-tion in with ion bombardment appears to beK2Cr2O7severe in comparison to the case, although theK2CrO4surface concentration may be a†ected by adsorption ofhydrocarbon in the initial stage of ion bombardment. Itmay again be stressed that the lower binding energypeak increases in the lower valence band peak due toion bombardment, suggesting that the surface is clearlyreduced to a conductive species.

In order to compare the degree of the reduction of

Figure 6. Chromium 2p XPS spectra from (a) cleavedK2Cr

2O

7:

in UHV; (b) exposed to air ; bombarded by argon ions for 60 s (c),120 s (d), 240 s (e) and 600 s (f).

Cr(VI) to Cr(III) by the argon ion bombardmentbetween and the concentration ofK2CrO4 K2Cr2O7 ,chromium in the surface of and isK2CrO4 K2Cr2O7plotted as a function of the ion bombardment time, asshown in Fig. 10. The chromium concentration isdivided into fractions of Cr(VI) and Cr(III), which maybe estimated in the following two ways : peak separationof Cr(VI) and Cr(III) in the Cr XPS spectra (the2p3@2fractions are denoted as dotted lines in Fig. 10) ;assuming that the presence of potassium a†ects the sta-bility of Cr(VI) in the two specimens, as recognized inFigs 8 and 9, so the amount of Cr(VI) may be correlatedwith that of potassium (fractions denoted as brokenlines in Fig. 10). The initial amount of Cr(VI) in Fig.10(b) appears to be inconsistent with the measuredchromium concentration. However, this is because asmall Cr(III) peak is observed in the cleaved K2Cr2O7 ,as given in Fig. 6(a), and the measured chromium con-centration is not equal to that of potassium, as shownin Fig. 9.

Finally, we will discuss a di†erence in the reductionrate of Cr(VI) by ion bombardment between K2CrO4and Bombardment of ions with high energiesK2Cr2O7 .may dissociate the chemical bonds of these compoundsmore or less, so that potassium is preferentially sput-

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 161È166 (1997)

CHEMICAL STATE CHANGES IN K2CrO4 AND K2Cr2O7 165

Figure 7. X-ray photoelectron spectra in the low-binding-energyrange from (a) cleaved in UHV; (b) exposed to air ;K

2Cr

2O

7:

bombarded by argon ions for 60 s (c), 120 s (d), 240 s (e) and600 s (f).

tered from the surface together with some oxygen. Thestability of oxides during the ion bombardment may beconcerned with the formation energy for the oxides.3 Infact, it is known that the formation energy for chro-mium oxide containing Cr(III) is lower than that foroxides containing Cr(VI),19,20 and this is consistent

Figure 8. Changes in the surface concentration of K, Cr and O inthe surface by argon ion bombardment.K

2CrO

4

Figure 9. Changes in the surface concentration of K, Cr and O inthe surface by argon ion bombardment.K

2Cr

2O

7

with the present results for the Cr(III) form in thesecompounds. Moreover, when the reduction of Cr(VI) isinduced by preferential sputtering of a given amount ofpotassium, the amount of Cr(III) formed in isK2Cr2O7twice as high as that for This appears to beK2CrO4 .responsible for the di†erence in the reduction ratebetween and as shown in Fig. 10.K2CrO4 K2Cr2O7 ,Since the e†ect of ion bombardment of the crystallo-graphic structures of orthorhombic and mono-K2CrO4clinic is not yet sufficiently revealed, it may beK2Cr2O7difficult to discuss a change in the atomic arrangementby ion bombardment. Nevertheless, we would maintainthe view that the preferential sputtering of potassium

Figure 10. The surface concentration of chromium in (a)K2CrO

4and (b) as a function of argon ion bombardment time.K

2Cr

2O

7The fractions of Cr(VI) and Cr(III) obtained from the fractions oftheir area in Cr 2p XPS spectra, and the variations in the potassiumconcentration, are denoted by dotted lines and broken lines,respectively.

( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 161È166 (1997)

166 S. SUZUKI, M. OKU AND Y. WASEDA

mainly induces the reduction of Cr(VI) to Cr(III) inand in the surface region by argonK2CrO4 K2Cr2O7ion bombardment.

CONCLUDING REMARKS

The XPS spectra from the clean surfaces of K2CrO4and were successfully obtained by cleavingK2Cr2O7their bulk specimens in UHV and optimizing conditionsof the neutralizer, and the e†ects of air exposure andargon ion bombardment on XPS spectra were studied.The main results are as follows :(1) The chemical state in the surfaces of andK2CrO4is little a†ected by air exposure, althoughK2Cr2O7the exposure alters the surface composition. On the

other hand, argon ion bombardment considerablyinduces the reduction of chromium ions from Cr(VI)to Cr(III) in and while it doesK2CrO4 K2Cr2O7 ,

not signiÐcantly change the chemical state of pot-assium and oxygen.

(2) It was found that preferential sputtering of pot-assium in and takes place duringK2CrO4 K2Cr2O7ion bombardment, and the degree of preferentialsputtering in is almost twice as high asK2Cr2O7that in K2CrO4 .

(3) The preferential sputtering of potassium stronglycorrelates with changes in the chemical state ofchromium ions, and the reduction behavior by ionbombardment can be mainly explained by prefer-ential sputtering of potassium from the surfaces of

andK2CrO4 K2Cr2O7 .

Acknowledgements

The present authors are grateful to Mr T. Sato and Ms F. Yasuda forhelp in operating the apparatus.

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( 1997 by John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 25, 161È166 (1997)