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Langmuir 1999, 15, 2806-2808
Correlation of Relative X-ray Photoelectron Spectroscopy Shake-up Intensity with CuO Particle Size C. C. Chusuei, M. A. Brookshier, and D. W. Goodman* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012 Received November 2, 1998 X-ray photoelectron spectroscopy (XPS) analysis of four Cu/SiO2 catalyst systems of different particle sizes of CuO on the surface showed variation in the relative peak area intensities of the shake-up lines to main core levels of the Cu 2p orbitals. These differences were attributed to various degrees of XPS-induced reduction of CuO initially formed on the surface by spin coating copper(II) acetate {Cu(CH3CO2)2‚H2O, Cu(ac)2} solutions of varying concentration. Changes in Cu L3M4,5M4,5 X-ray excited Auger (XAES) line shapes under time-dependent exposure to the soft Mg KR X-rays revealed that smaller particle sizes were more susceptible to reduction to Cu(+1) than larger ones. The degree of reduction of Cu(+2) to Cu(+1) correlated with measured atomic force microscopic (AFM) particle heights of CuO on these substrates prior to XPS.
Introduction Forming nanometer-sized metal particles on oxide surfaces by spin coating on flat surfaces is of interest both fundamentally and practically because it enables quantitative analysis of structure and composition of the surface (via surface science techniques) not readily accessible to porous supports that heterogeneous catalysts are typically impregnated into. Recently, work has addressed metallic Cu on Si wafers using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).1-3 We have reported elsewhere3 the utility of spin coating to control the CuO particle size in a Cu/SiO2 model catalyst via depositing varying concentrations of copper(II) acetate {Cu(CH3CO2)2‚H2O, Cu(ac)2} onto SiO2/Si(100) supports. Our AFM measurements showed that the CuO particle size could be effectively controlled to the order of 1 nm by varying solution concentrations at the nanometer scale. In performing XPS in conjunction with the AFM, we observed evidence for varying degrees of reduction of CuO on the surface from a Cu(+2) to a Cu(+1) state depending on solution preparation. Examination of XPS-induced reduction of these particles is the principle focus of this study. Reduction of Cu(+2) to Cu(+1) during XPS analysis has been widely reported in the literature and has been attributed to the integrated X-ray dose,4 heating from the X-ray gun,5 outgassing in a vacuum,6,7 and slow electrons emanating from the X-ray window during analysis.8 While the reduction of analyte in most cases is an unwanted experimental artifact, this property may be exploited to provide indirect measurements of the particle size of * To whom correspondence should be addressed. (1) Laszlo, C.; Wieldraaijer, W. Catal. Lett. 1993, 7, 71. (2) Partridge, A.; Toussaint, S. L. G.; Flipse, C. F. J. Appl. Surf. Sci. 1996, 103, 127. (3) Brookshier, M. A.; Chusuei, C. C.; Goodman, D. W. Langmuir 1999, in press. (4) Wallbank, B.; Johnson, C. E.; Main, I. G. J. Electron Spectrosc. Relat. Phenom. 1974, 4, 263. (5) Klein, J. C.; Li, C. P.; Hercules, D. M.; Black, J. F. Appl. Spectrosc. 1984, 38 (5), 729. (6) Hirokawa, K.; Honda, F.; Oku, M. J. Electron Spectrosc. Relat. Phenom. 1975, 6, 333. (7) Tobin, J. P.; Hirschwald, W.; Cunningham, J. Appl. Surf. Sci. 1983, 16, 441. (8) Iijima, Y.; Niimura, N.; Hiraoka, K. Surf. Interface Anal. 1996, 24, 193.
metallic Cu particles on the surface if the conditions are sufficiently controlled. We report a strong correlation between the relative amount of XPS-induced reduction of Cu(+2) to Cu(+1) and CuO particle size. Experimental Section A custom-designed apparatus consisting of a rotating metal support powered by an 18-V dc motor with a variable-speed potentiometer was used for spin coating.3 Contact-mode AFM was performed using a Digital Instruments Nanoscope II microscope. All AFM measurements were made in air using Nanoscope cantilevers with integrated Si3N4 pyramidal tips. XPS was performed in a Perkin-Elmer PHI 560 system using a PHI 25-270AR double-pass cylindrical mirror analyzer (CMA) with a Mg KR anode operated at 12 kV and 200 W with a photon energy of hν ) 1253.6 eV. A pass energy of 50 eV was used for high-resolution scans. Sputter-cleaned Cu 2p3/2 (932.7 eV) and Au 4f7/2 (84.0 eV) were used as standards to calibrate the BE range.9 The signal from adventitious carbon as a BE standard at 284.7 ( 0.2 eV was used to correct for charging on the substrate as described by Barr.10,11 The Cu(ac)2 solutions were prepared from a powder (Aldrich Chemical Co., 99.99+%), dissolving it in butanol. Si(100) wafers (Wafernet, Inc.) were cut into 1.0 cm × 1.0 cm pieces using a diamond scribe and used as flat supports for spin coating to prepare nanometer-sized CuO particles. Four model catalyst systems were prepared by varying Cu(ac)2 solution concentrations (0.010, 0.0085, 0.0070, and 0.0040 M), spin coating onto 5-10 nm thick SiO2 overlayers on Si(100) at 5000 rpm, and calcinating to 450 °C at a 300 °C/h ramp rate for 4 h as previously described.3 These solution concentrations produced CuO particles ranging from 3.7 to 6.3 nm in mean particle height as determined by AFM. The samples were then loaded into the XP spectrometer via an antechamber for XPS measurements. The operating system pressure during these measurements was ca. 1 × 10-9-1.0 × 10-8 Torr. High-resolution scans of the Cu 2p XPS and Cu L3M4,5M4,5 Auger regions were scanned. Peak line shapes and positions from X-ray excited Auger (XAES) have proved to be sensitive enough for Cu oxidation state analysis, providing more unambiguous differences between Cu(+1) and Cu(0) than Cu 2p XPS core level shifts.12 XAES of the Cu L3M4,5M4,5 region was (9) Seah, M. P. Surf. Interface Anal. 1989, 14, 488. (10) Barr, T. L. Modern ESCA; CRC Press, Inc.: Boca Raton, FL, 1994; Chapter 6 and references cited therein. (11) Barr, T. L.; Seal, S. J. Vac. Sci. Technol. A 1995, 13, 1239. (12) Strohmeier, B. R.; Leyden, D. E.; Field, R. S.; Hercules, D. M. J. Catal. 1985, 94, 514.
10.1021/la9815446 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/20/1999
XPS Shake-up Intensity with a CuO Particle Size
Figure 1. Stackplot of XP spectra of Cu 2p core levels of Cu(ac)2 in butanol spin-coated on oxidized Si(100) at 5000 rpm and calcinated to 450 °C at 300 °C/h for 4 h. acquired immediately upon initial X-ray irradiation. These substrates were then exposed to the soft X-rays for an additional 50 min followed by further sets of XAES scans.
Results and Discussion Prior to deposition of the nanometer-sized CuO particles on the SiO2/Si(100) substrate, AFM revealed no structural features on the flat surface.3 Particles, varying in size, were readily seen at the different Cu(ac)2 concentrations used prior to spin coating; these sizes were measured using an average particle height. Particle height is the most accurate means of measuring particle size using contactmode AFM; a full discussion of the limitations of contactmode AFM is given elsewhere.2 Figure 1 shows a stackplot of XP spectra of the Cu 2p core regions acquired from these surfaces after AFM imaging. Shake-up features at ∼945 and ∼965 eV for the Cu 2p3/2 and 2p1/2 core levels are evident and diagnostic of an open 3d9 shell of Cu(+2). The peak positions and relative intensities of the satellites from these levels are indicative of the presence of CuO at the surface.5,7,8,13-18 The relative intensities of the shakeup lines to the main core level of both the Cu 2p3/2 and 2p1/2 levels varied as a function of Cu(ac)2 solution concentration. The shake-up intensities denoting CuO on the surface were relatively more intense at higher Cu(ac)2 concentration. The peakfit of the Cu 2p3/2 core level revealed two binding energy states (with fwhm in parentheses) at 932.8 (1.91) and 933.8 (3.12) eV, which we assign to a Cu(0/+1) state and CuO, respectively. The binding energy region scanned to obtain these Cu 2p peaks (975-925 eV) took approximately 40 min to acquire. It was during this acquisition time that X-ray irradiation from XPS caused reduction of the CuO particles. For the smaller CuO particles, the ratio of exposed surface area to bulk is greater, which results in an overall increased dosage of X-ray irradiation and hence greater susceptibility to reduction. In addition, the presence of adventitious carbon obtained from treating these substrates in air likely enhanced reduction. From our XPS measurements of the (13) Frost, D. C.; Ishitani, A.; McDowell, C. A. Mol. Phys. 1972, 24 (4), 861. (14) Robert, T.; Bartel, M.; Offergeld, G. Surf. Sci. 1972, 33, 123. (15) Scho¨n, G. Surf. Sci. 1973, 35, 96. (16) Scrocco, M. Chem. Phys. Lett. 1979, 63 (1), 52. (17) McIntyre, N. S.; Sunder, S.; Shoesmith, D. W.; Stanchell, F. W. J. Vac. Sci. Technol. 1981, 18 (3), 714. (18) Ghijsen, J.; Tjeng, L. H.; van Elp, J.; Eskes, H.; Westerink, J.; Sawatzky, G. A.; Czyzyk, M. T. Phys. Rev. B 1988, 38 (16), 11322.
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Figure 2. XAES of Cu L3M4,5M4,5 transition of 0.0040 and 0.010 M Cu(ac)2 concentrations used to form small and large CuO particles on the SiO2/Si(100) surfaces, respectively: (a) immediate scans; (b) after a 50-min exposure to the Mg KR soft X-rays. The arrows denote the kinetic energy peak position at 917.8 eV.
C 1s level intensities and taking into account atomic sensitivity factors for all of the orbitals scanned (including O 1s, Cu 2p3/2, and Si 2p),19 there was 12-25 atom % carbon on these surfaces. XPS-induced reduction of Cu(+2) to Cu(+1) has been reported to increase in the presence of carbonaceous overlayers.4,13,20 To test this hypothesis of particle size dependent reduction rates, the Cu L3M4,5M4,5 XAES regions were examined before and after exposure of the substrates to the soft X-rays. Figure 2 shows Auger spectra of the Cu LMM transition of 0.010 and 0.0040 M concentrations of Cu(ac)2 spin-coated and calcinated on the SiO2/Si(100) substrates corresponding to the largest and smallest CuO particle sizes. Spectra a of both samples were taken immediately after initial X-ray irradiation and spectra b after an additional 50 min of exposure to the X-ray gun. Instrumental parameters and X-ray exposure times were identical for both particle sizes scanned. The arrows at 917.8 eV denote the XAES peak position of CuO, which is consistent with the literature.7,12,15 No difference in the Auger line shape or position was seen in the 0.010 M Cu(ac)2 prepared sample; however, a marked difference was seen for the 0.0040 M substrate before and after the 50-min X-ray exposure. Even upon initial exposure to the X-ray source, a difference in the Auger line shapes can be seen between the two particle sizes in spectra a. This difference in the Auger line shape between spectrum a of 0.0040 M Cu(ac)2 and that of 0.010 M Cu(ac)2 indicates that some reduction has already taken place during this initial scan. After 50 min, a feature at 916.8 eV in spectrum b of the 0.0040 M Cu(ac)2 substrate, which we assign to a Cu(+1) state (Cu2O),7,12,15,21 dominates. Some intensity within this spectral region can be seen in spectra a but is relatively weak compared to the 917.8 eV position. No intensity was observed within the 918.8 eV Cu L3M4,5M4,5 region corresponding to that reported for metallic Cu(0).17,22 We thus conclude that Cu(0) is not present on the surface and that the CuO particles are likely reduced (19) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Muilenburg, G. E., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1979. (20) Losev, A.; Kostov, K.; Tyuliev, G. Surf. Sci. 1989, 213, 564. (21) McIntyre, N. S.; Rummery, T. E.; Cook, M. G.; Owen, D. J. Electrochem. Soc. 1976, 123 (8), 164. (22) Kowalczyk, S. P.; Pollak, R. A.; McFeely, F. R.; Ley, L.; Shirley, D. A. Phys. Rev. B 1973, 8 (6), 2387.
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Figure 3. Curvefit of XPS Cu 2p3/2 core level. Peak 1 denotes the BE state for Cu(+1). Peaks 2-4 denote the intensity of Cu(+2).
to Cu2O. Clearly, there is greater reduction for the smaller particles. Figure 3 shows a representative peakfit of the Cu 2p3/2 core level and its corresponding shake-up satellites. This particular fit is for the 0.0040 M Cu(ac)2 treated sample that exhibited the largest core level intensity due to Cu(+1). Gaussian line shapes with a linear background were used to fit peak 1, which is assigned to the Cu(+1) state; peaks 2-4 are assigned to the Cu(+2) state. The curve-fitting parameters used are in agreement with those made by Iijima et al., who also observed varying amounts of reduction of CuO from X-ray irradiation on a Cu oxide substrate.8 The ratio of the sum of the areas of peaks 2-4 to that of peak 1 was used as a measure of the relative amount of Cu(+2) to Cu(+1) on the surface. As the Cu oxide particle size increases, the relative amount of XPSinduced reduction decreases. Intensity from Cu(+2) dominated in XP scans of larger particles (6.3 nm). Intensity from Cu(+1) dominated in XP scans of smaller ones (3.7 nm); in addition, there was a decrease in the Cu 2p shake-up intensity (Figure 1). Figure 4 shows a plot of [Cu2+]/[Cu+] calculated from the above-mentioned peak areas as a function of the AFM-measured cluster heights
Chusuei et al.
Figure 4. Plot of [Cu2+]/[Cu+] from XPS Cu 2p3/2 peak areas as a function of particle size measured using AFM cluster height (nm). These particles were assumed to be spherical.
of the CuO particles; [Cu2+]/[Cu+] varies linearly and increases with particle size. It should be emphasized that the AFM and XPS data in this figure were acquired from the same surfaces. In summary, there was an obvious correlation observed between the relative amount of soft X-ray-induced reduction from Cu(+2) to Cu(+1) from XPS and CuO particle size (as measured by AFM particle height) in the Cu/SiO2 model catalyst studied. Thus, we have demonstrated the utility of using XPS-induced reduction to indirectly measure CuO particle size. This methodology may prove to be useful in characterizing similar metallic particles of other model catalyst systems in which the analyte has pronounced XPS shake-up satellite features. Acknowledgment. We acknowledge with pleasure the support of this work by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, and the Robert A. Welch Foundation. C.C.C. gratefully acknowledges financial support from the Associated Western Universities, Inc., and Pacific Northwest Laboratories operated by Battelle Memorial. LA9815446
