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Evidence of photoreduction in thermal oxides formed on chromium
Materials Science and Engineering, A 103 (1988) L5-L7
L5
Letter
Evidence of photoreduction in thermal oxides formed on chromium G. P. HALADA and C. R. CLAYTON Department of Materials Science, State University of New York at Stony Brook, Stony Brook, NY 11794 (U.S.A.) D. H. LINDSLEY Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, N Y 11794 (U.S.A.) (Received April 25, 1988)
basis for the formation of CrO~ on the surface of C r 2 0 3.
Photoreduction effects resulting from the soft X-ray exposure of various compounds have been discussed previously [6-8]. In particular, De Angelis [8] observed surface reduction of CrO 3 during XPS analysis of powder samples during the first 30 min of X-ray exposure. In a later paper, Cimino et al. [9] attempted to correct for this reduction by taking spectra during the first 5 min of irradiation. No evidence of a C r 4 + photoreduction product has been reported. To date, the only observed or predicted photoreduction product of C r 6 + is Cr 3+
Abstract Cr ~+, formed as CrO~ along with Cr:O3 during the oxidation of pure chromium, is shown to reduce to Cr4+ (as CrO:) in the course of X-ray photoelectron spectroscopy analysis. Chromate which formed as a result of surface hydration of the oxide film was less affected by irradiation. Estimates have been made of the amounts of Cr03 and Cr042 present in the oxide film prior to irradiation.
2. Experimental procedure The equipment and methodology used in this work for chemical analysis by electron spectroscopy, as well as techniques for computer processing of data, have been described elsewhere [10]. The CrO2 peak binding energy and spectral characteristics were determined from a CrO 2
1. Introduction Oxide films formed on chromium in oxygen or air at temperatures up to 600 °C have been shown to be mainly composed of C r 2 0 3 , although in several cases oxide stoichiometries enriched in oxygen have been noted [1, 2]. Feve and coworkers [1] found evidence from X-ray photoelectron spectroscopy (XPS) analysis of Cr 6+ on the surface of a chromium sample heated to 400 °C in air, whilst chromium oxide studies by Gewinner et al. [3] indicated an overall chromium-to-oxygen ratio ranging from 0.52 to 0.66 at temperatures up to 500 °C. Watari [4] took steps to relate the excess oxygen to a possible interfacial or surface oxide region enriched in oxygen. In films formed by Watari [4] under ambient conditions, the chromium-oxygen stoichiometry most closely approximated to that of Cr203. Earlier, Brennan et al. [5] gave a thermodynamic 0921-5093/88/$3.50
Cr2P3/2
; BINDING ENERGY
Fig. 1. Cr 2p32 multiplet from the CrO~ standard. The singlets are Cr~ 2 (curve 1), Cr203 (curve 2)~ Cr(OH)3 (curve 3), CrO 3 (curve 4) and CrO42- (curve 5). © Elsevier Sequoia/Printed in The Netherlands
L6
5 min.
" 2",..~
"~" o°"
•,••i° °
~; 3 :' 1 .""", - ~ . . ~"::~'" °•
° °o .••
••
•~•
powder sample prepared by a high pressure method [11]. Equimolar amounts of dried C r 2 0 3 and f r O 3 w e r e placed in a platinum capsule which was hermetically sealed and held at 400 °C and 2.2 kbar pressure for 16 h. The product, a ferromagnetic and electrically conductive black powder, was determined by X-ray diffraction to be CrO2. In oxidation studies conducted in situ on pure chromium at 400 °C and in oxygen at 0.2 Torr, we have noted a surface species of C r O 3 (accompanied by a small amount of chromate) with u n d e r l y i n g C r 2 0 3 and CrO2, the latter having chromium in a 4 + oxidation state [12]. In comparison with this, films formed at lower temperatures seem to show little amounts of 4 + or 6 + valency oxides. 3. Results and discussion
A deconvoluted Cr 2p3/2 spectrum of the CrO 2 standard produced by the group is shown in Fig. 1. The spectrum was collected within the first 3 min of X-ray exposure in order to limit the radiation damage. Its position and shape agree with the singlet fitted as the decomposition product in all spectra. Cr 2p3/2 spectra from the in situ oxide film taken after 5 min, 40 min and 2 h MgKa/,2 400 W X-ray exposure show reductive behavior (Fig. 2). Table 1 is a summary of our results as they relate to the peak areas involved. Within the first 40 min of X-ray exposure, the C r O 3 peak decreased by nearly 50%, while the CrOE peak grew correspondingly. No other peak showed a change of the magnitude necessary to account for the change in area of the CrO3 peak, or for that matter for the reduction of the chromate peak. After 2 h of X-ray exposure the C r O 3 peak continued to reduce. The CrO2 peak no longer grew, whilst the C r 2 0 3 peak showed slight growth, per-
40 rain.
2 hr.
/
/
.--~ •
.,
2' ° "°',.
• •
1 ." "K..~_..~, •°'"
•.'" •..4"'"
TABLE 1 Oxide species
BINDING ENERGY Fig. 2. Cr 2p3/2 multiplets from the oxide showing the reduction as a function of irradiation time. The singlets are identified as in Fig. 1.
Cr
2P3/2singlet
p e a k areas as a p e r c e n t a g e of
the overall multiplet
CrO2
Binding Singlet peak areas (%) after exposure energy 5 rain 40 rain 2h (eV)
575.2 576.3 Cr(OH)3 577.0 CrO 3 578.3 frO4 2 579.6
Cr203
21.5 50.1 18.4 6.7 3.3
26.4 50.1 17.0 4.0 2.6
25.0 51.0 18.5 2.8 2.7
L7 30.0 o CrO 2
• CrO
o------___L
• CrO~ 2°
25.0
~0.0 Q IS.O u
lo.o
,-~ 5.0
0.0
J
0
20
i
40 Hlnutes
i_
6U 80 lug of X-ray Exposure
i
12U
!
140
Fig. 3. Line chart of singlet peak area reduction and growth vs. exposure time.
haps related to some minor photoreduction of C r O 2 t o the 3 + state. Figure 3 shows a line graph of the reduction with time of Cr 6+, as contained in CrO3 and CrO42-, t o f r O 2. In Fig. 4 we have reconstructed a hypothetical multiplet of Cr 2p3/2 which would result if no photoreduction of C r 6 + occurred. Decomposition resulting from heating of the sample by X-rays may, for the most part, be discounted owing to very low charging of the samples, which is indicative of good electrical and thermal contact with the sample holder. In addition to this, the decomposition temperatures for those species studied exceed the temperatures expected from photon heating of the sample surface. Such effects were noted, however, when the sample was annealed at temperatures above 150°C. Furthermore, CrO 3 powder samples cooled by liquid nitrogen continued to show evidence of photoreduction. 4.
Conclusions
(1) Considerably more Cr 6+ is actually present in the oxide films formed on chromium than is directly observed by XPS. (2) If the C r 4+ singlet is not taken into account in chromium peak analysis, then the binding energies and peak parameters of the trivalent and hexavalent singlets are bound to be in error. Acknowledgments This work was supported by the National Science Foundation under contract DMR-
i
BINDING
ENERGY
Fig. 4. Hypothetical reconstruction of Cr 2p3/2 multiplet showing no reduction of Cr ~+. The singletsare identifiedas stated in Fig. 1.
8418873 administered by Dr. Bruce MacDonald. The V.G.S. Electron Spectrometer and V.G.S. 1000 Data System were acquired from N.S.F. equipment awards DMR7718319 and DMR8117321.
References 1 L. Feve, R. Fontaine, J. Arsene, M. Lenglet and R. Caillat, C.R. Acad. Sci. Paris, 301 (10)(1985) 701. 2 G.A. Hope and I. M. Ritchie, Thin Solid Films, 34 (1976) 111. 3 G. Gewinner, J. C. Peruchetti, A. Jaegle and A. Kalt, Surf Sci., 18(1978) 439. 4 F. Watari, Thin Solid Films, 97(1982) 31. 5 D. Brennan, D. O. Hayward and B. Trapnell, Proc. R. Soc. London, Ser. A, 256(1960) 81. 6 S. Storp, Spectrochim. Acta, PartB, 40(516)(1985) 745. 7 R. G. Copperthwaite, Surf. Interface AnaL, 2 (1) (1980) 17. 8 B.A. De Angelis, J. Electron Spectrosc., 9 (1976) 81. 9 A. Cimino, B. A. De Angelis, A. Luchetti and G. Minelli, J. CataL, 45(1976) 316. 10 C.R. Clayton and Y. C. Lu, J. Electrochem. Soc.. 133(12) (1986) 2465. 11 G. P. Halada, C. R. Clayton and D. H. Lindsley, to be published. 12 G.P. Halada and C. R. Clayton, to be published.
L5
Letter
Evidence of photoreduction in thermal oxides formed on chromium G. P. HALADA and C. R. CLAYTON Department of Materials Science, State University of New York at Stony Brook, Stony Brook, NY 11794 (U.S.A.) D. H. LINDSLEY Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, N Y 11794 (U.S.A.) (Received April 25, 1988)
basis for the formation of CrO~ on the surface of C r 2 0 3.
Photoreduction effects resulting from the soft X-ray exposure of various compounds have been discussed previously [6-8]. In particular, De Angelis [8] observed surface reduction of CrO 3 during XPS analysis of powder samples during the first 30 min of X-ray exposure. In a later paper, Cimino et al. [9] attempted to correct for this reduction by taking spectra during the first 5 min of irradiation. No evidence of a C r 4 + photoreduction product has been reported. To date, the only observed or predicted photoreduction product of C r 6 + is Cr 3+
Abstract Cr ~+, formed as CrO~ along with Cr:O3 during the oxidation of pure chromium, is shown to reduce to Cr4+ (as CrO:) in the course of X-ray photoelectron spectroscopy analysis. Chromate which formed as a result of surface hydration of the oxide film was less affected by irradiation. Estimates have been made of the amounts of Cr03 and Cr042 present in the oxide film prior to irradiation.
2. Experimental procedure The equipment and methodology used in this work for chemical analysis by electron spectroscopy, as well as techniques for computer processing of data, have been described elsewhere [10]. The CrO2 peak binding energy and spectral characteristics were determined from a CrO 2
1. Introduction Oxide films formed on chromium in oxygen or air at temperatures up to 600 °C have been shown to be mainly composed of C r 2 0 3 , although in several cases oxide stoichiometries enriched in oxygen have been noted [1, 2]. Feve and coworkers [1] found evidence from X-ray photoelectron spectroscopy (XPS) analysis of Cr 6+ on the surface of a chromium sample heated to 400 °C in air, whilst chromium oxide studies by Gewinner et al. [3] indicated an overall chromium-to-oxygen ratio ranging from 0.52 to 0.66 at temperatures up to 500 °C. Watari [4] took steps to relate the excess oxygen to a possible interfacial or surface oxide region enriched in oxygen. In films formed by Watari [4] under ambient conditions, the chromium-oxygen stoichiometry most closely approximated to that of Cr203. Earlier, Brennan et al. [5] gave a thermodynamic 0921-5093/88/$3.50
Cr2P3/2
; BINDING ENERGY
Fig. 1. Cr 2p32 multiplet from the CrO~ standard. The singlets are Cr~ 2 (curve 1), Cr203 (curve 2)~ Cr(OH)3 (curve 3), CrO 3 (curve 4) and CrO42- (curve 5). © Elsevier Sequoia/Printed in The Netherlands
L6
5 min.
" 2",..~
"~" o°"
•,••i° °
~; 3 :' 1 .""", - ~ . . ~"::~'" °•
° °o .••
••
•~•
powder sample prepared by a high pressure method [11]. Equimolar amounts of dried C r 2 0 3 and f r O 3 w e r e placed in a platinum capsule which was hermetically sealed and held at 400 °C and 2.2 kbar pressure for 16 h. The product, a ferromagnetic and electrically conductive black powder, was determined by X-ray diffraction to be CrO2. In oxidation studies conducted in situ on pure chromium at 400 °C and in oxygen at 0.2 Torr, we have noted a surface species of C r O 3 (accompanied by a small amount of chromate) with u n d e r l y i n g C r 2 0 3 and CrO2, the latter having chromium in a 4 + oxidation state [12]. In comparison with this, films formed at lower temperatures seem to show little amounts of 4 + or 6 + valency oxides. 3. Results and discussion
A deconvoluted Cr 2p3/2 spectrum of the CrO 2 standard produced by the group is shown in Fig. 1. The spectrum was collected within the first 3 min of X-ray exposure in order to limit the radiation damage. Its position and shape agree with the singlet fitted as the decomposition product in all spectra. Cr 2p3/2 spectra from the in situ oxide film taken after 5 min, 40 min and 2 h MgKa/,2 400 W X-ray exposure show reductive behavior (Fig. 2). Table 1 is a summary of our results as they relate to the peak areas involved. Within the first 40 min of X-ray exposure, the C r O 3 peak decreased by nearly 50%, while the CrOE peak grew correspondingly. No other peak showed a change of the magnitude necessary to account for the change in area of the CrO3 peak, or for that matter for the reduction of the chromate peak. After 2 h of X-ray exposure the C r O 3 peak continued to reduce. The CrO2 peak no longer grew, whilst the C r 2 0 3 peak showed slight growth, per-
40 rain.
2 hr.
/
/
.--~ •
.,
2' ° "°',.
• •
1 ." "K..~_..~, •°'"
•.'" •..4"'"
TABLE 1 Oxide species
BINDING ENERGY Fig. 2. Cr 2p3/2 multiplets from the oxide showing the reduction as a function of irradiation time. The singlets are identified as in Fig. 1.
Cr
2P3/2singlet
p e a k areas as a p e r c e n t a g e of
the overall multiplet
CrO2
Binding Singlet peak areas (%) after exposure energy 5 rain 40 rain 2h (eV)
575.2 576.3 Cr(OH)3 577.0 CrO 3 578.3 frO4 2 579.6
Cr203
21.5 50.1 18.4 6.7 3.3
26.4 50.1 17.0 4.0 2.6
25.0 51.0 18.5 2.8 2.7
L7 30.0 o CrO 2
• CrO
o------___L
• CrO~ 2°
25.0
~0.0 Q IS.O u
lo.o
,-~ 5.0
0.0
J
0
20
i
40 Hlnutes
i_
6U 80 lug of X-ray Exposure
i
12U
!
140
Fig. 3. Line chart of singlet peak area reduction and growth vs. exposure time.
haps related to some minor photoreduction of C r O 2 t o the 3 + state. Figure 3 shows a line graph of the reduction with time of Cr 6+, as contained in CrO3 and CrO42-, t o f r O 2. In Fig. 4 we have reconstructed a hypothetical multiplet of Cr 2p3/2 which would result if no photoreduction of C r 6 + occurred. Decomposition resulting from heating of the sample by X-rays may, for the most part, be discounted owing to very low charging of the samples, which is indicative of good electrical and thermal contact with the sample holder. In addition to this, the decomposition temperatures for those species studied exceed the temperatures expected from photon heating of the sample surface. Such effects were noted, however, when the sample was annealed at temperatures above 150°C. Furthermore, CrO 3 powder samples cooled by liquid nitrogen continued to show evidence of photoreduction. 4.
Conclusions
(1) Considerably more Cr 6+ is actually present in the oxide films formed on chromium than is directly observed by XPS. (2) If the C r 4+ singlet is not taken into account in chromium peak analysis, then the binding energies and peak parameters of the trivalent and hexavalent singlets are bound to be in error. Acknowledgments This work was supported by the National Science Foundation under contract DMR-
i
BINDING
ENERGY
Fig. 4. Hypothetical reconstruction of Cr 2p3/2 multiplet showing no reduction of Cr ~+. The singletsare identifiedas stated in Fig. 1.
8418873 administered by Dr. Bruce MacDonald. The V.G.S. Electron Spectrometer and V.G.S. 1000 Data System were acquired from N.S.F. equipment awards DMR7718319 and DMR8117321.
References 1 L. Feve, R. Fontaine, J. Arsene, M. Lenglet and R. Caillat, C.R. Acad. Sci. Paris, 301 (10)(1985) 701. 2 G.A. Hope and I. M. Ritchie, Thin Solid Films, 34 (1976) 111. 3 G. Gewinner, J. C. Peruchetti, A. Jaegle and A. Kalt, Surf Sci., 18(1978) 439. 4 F. Watari, Thin Solid Films, 97(1982) 31. 5 D. Brennan, D. O. Hayward and B. Trapnell, Proc. R. Soc. London, Ser. A, 256(1960) 81. 6 S. Storp, Spectrochim. Acta, PartB, 40(516)(1985) 745. 7 R. G. Copperthwaite, Surf. Interface AnaL, 2 (1) (1980) 17. 8 B.A. De Angelis, J. Electron Spectrosc., 9 (1976) 81. 9 A. Cimino, B. A. De Angelis, A. Luchetti and G. Minelli, J. CataL, 45(1976) 316. 10 C.R. Clayton and Y. C. Lu, J. Electrochem. Soc.. 133(12) (1986) 2465. 11 G. P. Halada, C. R. Clayton and D. H. Lindsley, to be published. 12 G.P. Halada and C. R. Clayton, to be published.