Knowledge Base

XPS of gold is typically performed on the 4f region. Gold is often used to calibrate XPS instrumentation and the binding energy of pure gold according to ISO 15472:2010 is 83.95 eV.¹ The doublet separation between Au 4f_7/2 and 4f_5/2 is 3.67 eV. Significant overlap between Mg 2s and Au 4f renders deconvolution of Au/Mg systems technically challenging, however it may be achieved through careful interpretation of the Au 4f_7/2 peak.² Zn 3p may also cause a slight difficulty in peak fitting depending on the relative content of Au:Zn.

Figure 1: Survey spectrum of sputter cleaned gold foil³

Certain gold states may reduce under the X-ray beam⁴ and hence care should be taken during analysis – take a single spectrum before and after analysis to monitor potential reduction.

Figure 2: Au NP during (green/pink) X-ray irradiation, evidencing twin states and reducible behaviour. Red/green = fully reduced Au NPs.⁴

Small Au nanoparticles (< 10nm) may show significant binding energy shifts compared with bulk Au.⁵ Such shifts often appear to lower binding energies and are dependent on the support matrix and pretreatment conditions. It is commonly believed that this shift is due to (i) changes in electronic structure with cluster size, (ii) electron transfer from the support (and subsequent negative charging of the particles), (iii) modifications to the particle-support interactions by pre-treatments.^5,6 As such, the influence of final and initial state effects should be considered in all cases.

Typical binding energies for Au species may be found in table 1:

Table 1: Typical XPS binding energies for common Au species

References

1. ISO 15472:2010(en)Surface chemical analysis — X-ray photoelectron spectrometers — Calibration of energy scales, www.iso.org, accessed: 22/06/2020
2. Ardemani, L., et al. (2015). “Solid base catalysed 5-HMF oxidation to 2, 5-FDCA over Au/hydrotalcites: fact or fiction?” Chemical science 6(8): 4940-4945. Read it online here.
3. Data acquired by HarwellXPS
4. Doherty, S., et al. (2019). “Highly Selective and Solvent-Dependent Reduction of Nitrobenzene to N-Phenylhydroxylamine, Azoxybenzene, and Aniline Catalyzed by Phosphino-Modified Polymer Immobilized Ionic Liquid-Stabilized AuNPs.” ACS Catalysis 9(6): 4777-4791. Read it online here.
5. Radnik, J., et al. (2003). “On the origin of binding energy shifts of core levels of supported gold nanoparticles and dependence of pretreatment and material synthesis.” Physical Chemistry Chemical Physics 5(1): 172-177. Read it online here.
6. Sankar, M., et al. (2012). “Synthesis of stable ligand-free gold–palladium nanoparticles using a simple excess anion method.” ACS nano 6(8): 6600-6613. Read it online here.
7. Krozer, A. and M. Rodahl (1997). “X-ray photoemission spectroscopy study of UV/ozone oxidation of Au under ultrahigh vacuum conditions.” Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 15(3): 1704-1709. Read it online here.
8. Data recorded in-house at Cardiff Hub