Cu(0):Cu(II) or Cu(I):Cu(II) Calculations

The presence of the well known shake-up satellite found in Cu 2p spectra as an indication of the presence of Cu(II) species is well known. Recently, a study of the surface chemistry of the flotation separation of chalcocite (Cu2S) from heazelwoodite (Ni3S2) employed a fitting procedure and calculation that quantifies the amount of Cu(II) species present on the surface of Cu(I) sulfide[1] as first developed by Jasieniak and Gerson (2004) and now described in reference [2]. The calculation takes into account the photoelectron yield from both the main 2p3/2 peak and the shake-up peak and is based on main peak/shake-up peak ratios derived from Cu(OH)2 standard spectra.

Quantification of the amount of Cu(II) species on a Cu(0) or Cu(I) containing surface does appear to be possible. If, for example, a Cu metal surface is oxidized to Cu(II), the shake-up structure associated with the Cu(II) species can be used for a Cu(0):Cu(II) quantification. Alternatively if Cu(II) species and Cu(I) species are present, the Cu(I):Cu(II) ratio can be determined. This method of Cu(0):Cu(II) (or Cu(I):Cu(II)) determination depends on shake-up peaks that are present in the spectra of d9 Cu(II) containing samples but are absent in d10 Cu(0) (or Cu(I)) spectra. Shake-up peaks may occur when the outgoing photoelectron simultaneously interacts with a valence electron and excites it to a higher-energy level. The kinetic energy of the shaken-up core electron is then slightly reduced giving a satellite structure a few eV below (higher on the calculated BE scale) the core level position[3]. Hence, these electrons are part of the total Cu 2p emission and should be included in both total Cu and relative chemical state speciation. For example, the main emission line (A) in Figure 1 contains both Cu(II) (A1) and Cu(0) (A2) contributions but the satellite intensity (B) is entirely from Cu(II). The total intensity from Cu(II) species is represented in the combination of the signals from the direct photoemission (A1) and the shaken-up photoemission (B).

Accurate Cu(0):Cu(II) ratios for samples containing a mixture of Cu(0) and Cu(II) rely on determining an accurate ratio of the main peak /shake-up peak areas (A1s/Bs) for a 100% pure Cu(II) sample. With a reliable value of A1s/Bs obtained for Cu(OH)2 or CuO (where all copper present is in the Cu(II) state), the relative concentrations of Cu(0) and Cu(II) species present on a surface that contains both species can be obtained by the following simple equations[4]:

% Cu(0) = A2/(A+B)*100 = (A-A1)/(A+B)*100 = (A-(A1s/Bs)B)/(A+B)*100

% Cu(II) = (B+A1)/(A+B)*100 = B(1+(A1s/Bs))/(A+B)*100

where B is the area of the shake-up peak and A is the total area of the main peak.

In order to determine accurate values of A1s/Bs, seven Cu 2p3/2 analyses of pure Cu(OH)2 were obtained. Analyses were carried out on the various Cu(OH)2 samples at acquisition times of generally less than a few minutes as it has been shown that reduction of Cu(OH)2 can occur after extended X-ray exposure[5]. Our studies suggest that after X-ray exposures of 3 h, up to 10% of Cu(OH)2 has been reduced to Cu(I). At pass energies of 20 eV and 40 eV, A1s/Bs values of 1.57±0.1 and 1.59±0.1 were found, respectively. A similar analysis of a pure CuO sample was also carried out and gave a A1s/Bs value of 1.89±0.08 (20 eV pass energy). Figure 1 shows spectra for a sputter cleaned metal surface, CuO and Cu(OH)2 standards used for A1s/Bs determination and a spectrum of the native oxide on a pure metal surface with the amount of oxidation of the surface calculated[4].

It should be noted that the peak-shape and main peak to shake-up peak separation is quite different for Cu(OH)2 and CuO (Figure 1). This is useful (along with the O 1s signal if only Cu species are present) in determining which A1s/Bs value to use for Cu(0):Cu(II) (or Cu(I):Cu(II)) calculations. If the Cu(0) or Cu(I) signal is relatively strong, (and the sample is conducting) some assessment of which is present in the sample may be made based on the BE of the 2p3/2 peak[4].


An excel spreadsheet calculator that uses the equations above for Cu(II) determinations can be found here.


Figure 1. Cu 2p spectra for a sputter cleaned Cu metal surface (bottom), Cu2O standard (2nd from bottom, a small amount of Cu(II) was found in this sample), CuO standard (3rd from bottom), Cu(OH)2 standard (4th from bottom) used for A1s/Bs determination and a spectrum of a native oxide on a metal surface (top) with the proportion of Cu(0) and Cu(II) calculated [4].

References:
[2] A.R. Gerson, M. Jasieniak, The Effect of Surface Oxidation on the Cu Activation of Pentlandite and Pyrrhotite, in: W.D. Duo, S.C. Yao, W.F. Liang, Z.L. Cheng; Long H. (Eds.), Proceedings of the XXIV International Minerals Processing Congress, Science Press Beijing, Beijing, China, 2008, pp. 1054-1063.
[3] J.F. Watts, J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES, Wiley, Rexdale (2003) 71.
[5] W.M. Skinner, C.A. Prestidge, R.St.C. Smart, Surf. Interface Anal. 24 (1996) 620.