X-ray Degradation of Cu(OH)2

X-ray induced degradation of copper (II) species can complicate interpretation of results. One way to mitigate this issue is to perform the analysis of the Cu 2p (and Cu LMM Auger line) first and in as few scans as possible, then perform subsequent needed analysis afterward (e.g. other high resolution spectra, survey scans).  If the mechanism of reduction is due to thermal effects, as it appears to be for Cu(OH)2, cooling of the material can reduce the amount of degradation significantly. The charts below for copper (II) hydroxide samples analyzed by XPS at normal operating temperatures (top) and cooled to -100C (bottom) using the same X-ray source (15 kV, 14 mA, monochromatic Al K(alpha)) and charge neutralizer conditions (Kratos AXIS Ultra system) show that degradation is slowed significantly for the cooled sample.  Notably, degradation is minimal for the cooled sample for the initial window of analysis. 

X-ray induced degradation of Cu(OH)2 - normal operating temperatures.

X-ray induced degradation of Cu(OH)2 - sample cooled to -100C.

Video: Advanced Analysis of Copper XPS Spectra

2020 Kratos North American User Meeting talk by Dr. Mark Biesinger, Director of Surface Science Western at Western University, London, Ontario, Canada.  Various strategies for the analysis of Cu XPS (X-ray photoelectron spectroscopy) spectra.

Advanced Analysis of Copper X-ray Photoelectron Spectra

This recent paper [1] builds upon and extends previously published X-ray photoelectron spectroscopy (XPS) curve-fitting and data analysis procedures [2,3] for a wide range of copper containing species. Steps undertaken include: 1) an examination of existing Cu 2p3/2 main peak and Cu 2p3/2 - Cu L3M4,5M4,5 Auger parameter literature data, 2) analysis of a series of quality standard samples, 3) curve-fitting procedures for both the Cu 2p3/2 and the Cu L3M4,5M4,5 spectra (as well as associated anions), 4) calculations that determine the amount of Cu(II) species in a mixed oxidation state system, 5) calculations and necessary data for thin film mixed oxide/hydroxide thickness measurements and 6) a presentation of literature and standard sample values in a Wagner (chemical state) plot.

Some examples of the extensive datasets and spectra available in this paper are presented below.

Cu 2p3/2 spectra of various Cu(II) species.

Cu L3M4,5M4,5 spectra for (left) Cu(0), Cu(I) species, mineral samples, and (right) Cu(II) species.

Curve-fitted Cu L3M4,5M4,5 spectrum of wrought copper sample submerged in an Ar aerated 3M NaCl solution for 30 days.  Curve-fitting results suggest 22 % Cu(0), 65 % Cu2O and 13% Cu2S.

Cu 2p3/2 - Cu L3M4,5M4,5 Wagner (chemical state) plot with literature and standard sample data.

Cu 2p3/2 and Cu 2p3/2 - Cu L3M4,5M4,5 Auger parameter literature values from Cu species

Cu 2p3/2 (main peak only) and Cu 2p3/2 - Cu L3M4,5M4,5 Auger parameter values from standard samples (20 eV pass energy). a) 932.63 eV for non-monochromatic Al X-ray source, 932.62 eV for monochromatic Al K(alpha) X-ray source b) Note that this value is specified for the Kratos instruments used in this work. Other instruments may not have this accuracy. This will also not apply for Cu(0) in a non-conductive environment where calibration to other peaks are needed (e.g. adventitious carbon C 1s which has an error of 0.1 to 0.2 eV associated with it).

Reference:
[1] M.C. Biesinger, Advanced Analysis of Copper X-ray Photoelectron (XPS) Spectra, Surf. Interface Anal. 49 (2017) 1325-1334.
[2] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2010) 887-898.
[3] M.C. Biesinger, B.R. Hart, R. Polack, B.A. Kobe, R.St.C. Smart, Miner. Eng. 20 (2007) 152.

Copper Sulphides, Cu2S and CuS

Table 1. Cu 2p3/2 binding energies, Cu 2p3/2 - Cu LMM modified Auger parameters and S 2p3/2 binding energies for copper sulphides (Cu2S and CuS)[1].

CuS does not show shakeup peaks like CuO. CuS is weakly paramagnetic and behaves like a more metallic like species.

Further values for Cu2S (in vacuum cleaved chalcocite)[2].
Cu 2p3/2 = 932.5 eV
S 2p3/2 = 161.8 eV

CuInSe2 Auger Parameter: 1849.4 eV - 1849.6 eV [3]

References:
[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.
[2] M.C. Biesinger, B.R. Hart, R. Polack, B.A. Kobe, R.St.C. Smart, Miner. Eng. 20 (2007) 152.

[3] D. Cahen, P.J. Ireland, L.L. Kazmerski, F.A. Thiel, J. Appl. Phys. 57 (1985) 4761.

Cu LMM Peak Shapes

The Cu LMM peak shape (in addition to the Auger parameter and Cu 2p3/2 peak position and shape) can also be useful in determining Cu chemical states. It is particularly useful when determining Cu metal versus Cu (I) in the absence of Cu (II) species. Comparison of unknown spectra to the standard Cu LMM spectra can be used to estimate the relative amounts of each.

Figure 1. Cu LMM Auger spectra for Cu metal (top, left), Cu2O (top, right), CuO (bottom, left) and Cu(OH)2 (bottom, right) [1].

[1] M.C. Biesinger, unpublished data (2013).

UPDATE:
New work published using Cu LMM spectra - see:
M.C. Biesinger, Advanced Analysis of Copper X-ray Photoelectron (XPS) Spectra, Surface and Interface Analysis, 49 (2017) 1325-1334.

Copper

Table 1 lists Cu 2p_3/2 BE and modified Auger parameter values from a survey of literature sources compiled in the NIST Database[1]. Of note here is the statistically similar BE values for the Cu metal and Cu(I) oxide species. The use of the modified Auger parameter (2p_3/2, L₃M₄₅M₄₅) as well as an inspection of the Auger peak-shape do allow for a more accurate assignment for these species and has been used effectively. Goh et al.[2] have shown (in their Figure 8) the distinctly different peak shapes of the X-ray generated Auger LMM spectra for copper as the metal, Cu₂S and CuS. They also note the distinctive Cu L₃M_4,5M_4,5 peak at 916.5 eV for Cu₂O. Poulston et al.[3], in their study of surface oxidation and reduction of Cu₂O and CuO, have used both the Cu LMM and the Auger parameter to distinguish Cu(0), Cu(I) and Cu(II). These parameters are very useful for identification of the different states present in the surface but they are difficult to quantify as relative amounts of each species. The Cu 2p XPS spectrum is still the signal most used for this purpose.

Table 2 shows similar results to those shown in Table 1 from our work [4] for a series of standard samples. In this analysis, a statistical separation of the Cu 2p_3/2 peak position for Cu(0) and Cu₂O is achieved. This should be expected, as most spectrometer calibration procedures include referencing to the ISO standard Cu metal line at 932.62 eV for monochromatic Al K(alpha) or 932.63 eV for non-monochromatic Al K(alpha) and Mg K(alpha), with deviation of this line set at ±0.025 eV. Curve-fitting of the Cu 2p_3/2 line for both Cu metal and Cu₂O employed Gaussian (10 %) – Lorentzian (90 %) p and Gaussian (20 %) – Lorentzian (80 %) peak-shapes, respectively (defined in CasaXPS as GL(90) and GL(80)). Peak-shapes for these species are shown in Figure 1.

In practice, quantifying a mix of Cu(0), Cu(I) and Cu(II) species would require precise constraints on BE, FWHM, and peak-shape parameters. Resolution of these components will be difficult with larger amounts of Cu(II) compounds present due to the overlap of peaks for these three components. It may be possible to fit the 2p_3/2 spectrum using a set of constrained peaks that simulate the entire peak-shape (including the shake-up components) for the Cu(II) species present (Table 3) [4].

Table 1. Cu 2p_3/2 and modified Auger parameter literature values for Cu species (compiled from reference [1]).

Table 2. Cu 2p_3/2 and modified Auger parameter values for Cu species from [4]. [a) 932.63 eV for non-monochromatic Al X-ray source, 932.62 eV for monochromatic Al K(alpha) X-ray source]  

Table 3. Cu 2p_3/2 fitting parameters for Cu(II) species [4].

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) and Cu(OH)₂ standard (top) [4].

An advanced analysis of copper XPS spectra is now available here and in reference [5].

References:

[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.

[2] S.W. Goh, A.N. Buckley, R.N. Lamb, R.A. Rosenberg, D. Moran, Geochim. Cosmochim. Acta 70 (2006) 2210.

[3] S. Poulston, P.M. Parlett, P. Stone, M. Bowker, Surf. Interface Anal. 24 (1996) 811.

[4] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2010) 887-898.

[5] M.C. Biesinger, Surf. Interface Anal. 49 (2017) 1325-1334.

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[2] and now described in reference [3] and [4]. The calculation takes into account the photoelectron yield from both the main 2p_3/2 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[5]. 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 (A1_s/B_s) for a 100% pure Cu(II) sample. With a reliable value of A1_s/B_s 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[3,4]:

% Cu(0) = A2/(A+B)*100 = (A-A1)/(A+B)*100 = (A-(A1_s/B_s)B)/(A+B)*100

% Cu(II) = (B+A1)/(A+B)*100 = B(1+(A1_s/B_s))/(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 A1_s/B_s, seven Cu 2p_3/2 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[6]. 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, A1_s/B_s 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 A1_s/B_s 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 A1_s/B_s determination and a spectrum of the native oxide on a pure metal surface with the amount of oxidation of the surface calculated[3,4].

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 A1_s/B_s 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 2p_3/2 peak[3,4].

An excel spreadsheet calculator that uses the equations above for Cu(II) determinations can be found here. (Note: you must download the file to Excel to use it - it is locked in Google Docs).

Figure 1. Cu 2p spectra for a sputter cleaned Cu metal surface (bottom), Cu₂O standard (2^nd from bottom, a small amount of Cu(II) was found in this sample), CuO standard (3^rd from bottom), Cu(OH)₂ standard (4^th from bottom) used for A1_s/B_s determination and a spectrum of a native oxide on a metal surface (top) with the proportion of Cu(0) and Cu(II) calculated [3,4].

References:

[1] M.C. Biesinger, B.R. Hart, R. Polack, B.A. Kobe, R.St.C. Smart, Miner. Eng. 20 (2007) 152.

[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] M.C. Biesinger, L.W.M. Lau, A.R. Gerson, R.St.C. Smart, Appl. Surf. Sci. 257 (2010) 887-898.

[4] M.C. Biesinger, Surf. Interface Anal. 49 (2017) 1325-1334.

[5] J.F. Watts, J. Wolstenholme, An Introduction to Surface Analysis by XPS and AES, Wiley, Rexdale (2003) 71.

[6] W.M. Skinner, C.A. Prestidge, R.St.C. Smart, Surf. Interface Anal. 24 (1996) 620.