WO3 may not be stable above 484 deg C (refer to the W-O phase diagram).

WnO3n-2 phase could form at high temperatures - typically WO3 containing oxygen vacancies. Even though you are annealing in air at 600 C, there is a good chance that you form a significant concentration of oxygen vacancies. For charge neutrality, you have to convert some of the W6+ to W5+ or to W4+. When doped with CuO, the oxygen vacancy formation could result in creation of lower valence state Cu in addition to converting some of the W6+ to W5+. In order to verify this reaction mechanism, you have to compare the XPS spectra of undoped and CuO doped WO3 that were annealed in air at 600 C. You may see that the amount of W5+ is less in the CuO doped WO3.

Thank you sir

Kavitha V S The surface (5 - 10 atomic layers) of every material in air when investigated by XPS will show high oyygen content, the element in its highest oxidation state (in your case Cu2+) and a large amount of carbon at 285 eV. The latter is used to calibrate the system taking into account charging. You need to argon ion etch 10 or more atomic layer to see the bulk state of the material. From what you state, it appears that there is some reduction of your CuO by the WO3. This could be a catalyzed decomposition of CuO as well:

3CuO → Cu2O + Cu0+ O2

600 C is a very high annealing' stage. Why did you use this temperature? The phase diagram of copper and oxygen would be useful to study. See attached. Here you see, depending on the partial pressure of oxygen (and WO3 must affect this), a variety of forms of copper can be seen. Indeed from the second attachment we see that Cu is the preferred state (i.e. CuO decomposes) at higher partial pressures of O2 at 600 C.

Reference: DOI:10.1016/S0021-8502(00)90563-9

Indeed the 3rd attachment shows that all forms of Cu can be present at 600 C.

Cu later....

Also take a look at: 'Thermogravimetric study of the reduction of CuO–WO3 oxide mixtures in the entire range of molar ratios' Jelić, D., Zeljković, S., Škundrić, B. et al. J Therm Anal Calorim (2018) 132: 77. https://doi.org/10.1007/s10973-017-6921-0

That means that, CuO doping of WO3 resulted in mixed oxidation for Cu, despite being annealed in air up tp 600C.

Engin Ciftyurek Yes, but why is that an issue or problem, as you seem to hint?

What the acquisition time for your of XPS spectra? It is well known that X-ray exposure during acquisition of XPS spectra induces reduction of CuO to

Cu2O. This has been thoroughly studied in the literature and has been attributed to the integrated X-ray dose, heating from the X-ray gun, outgassing in a vacuum and slow electrons emanating from the X-ray window during analysis.

Some useful references:

1. B. Wallbank, C.E. Johnson, I.G. Main, J. Electron Spectrosc. Relat.

Phenom. 4, 263 (1974)

2. J.C. Klein, C.P. Li, D.M. Hercules, J.F. Black, Appl. Spectrosc.

38, 729 (1984)

3. T.P. Tobin, W. Hirschwald, J. Cunningham, Appl. Surf. Sci. 16,

441 (1983)

4. Y. Iijima, N. Niimura, K. Hiraoka, Surf. Interface Anal. 24, 193

(1996)

5. C.C. Chusuei, M.A. Brookshier, D.W. Goodman, Langmuir 15,

2806 (1999)

6. M. Naddaf · O. Mrad · A. Al-zier, Appl Phys A (2014) 115:1345–1353

Thank you all for your valuable comments