Charge transfer at interfaces of copper clusters on TiO2(110) and SiOx

Metal nanoparticles supported on different oxidic supports are the most common materials in heterogeneous (photo-)catalysis. This work presents a systematic investigation of copper clusters deposited onto slightly and highly reduced rutile TiO₂(110) single crystals and silicon wafers with native oxide films. The focus is on the electronic properties of the copper clusters and possible metal-support interactions as these can change the catalytic behavior of the catalyst. Specifically, we examine coverage-dependent core-level binding energy shifts and kinetic energy Auger signal shifts of the Cu2p_3/2 and CuLMM signals in X-ray photoelectron spectroscopy as well as a Wagner plot analysis, Auger parameter analysis, and analyze the main support signals. The final-state-induced binding energy shifts dominant at lower coverages are related to the imperfect core-hole shielding of the positive charge remaining after photoemission. At higher copper coverages the more metallic character of the clusters, apparent from dominating initial-state effects, is more prominent. The shift in binding energy, kinetic energy, and Auger parameter are larger for copper on silica than for copper on reduced titania. The formation of Ti³⁺ or Si³⁺ indicates a charge transfer from the metal clusters to the support. For the first nominal monolayer of copper on titania a constant number of Ti³⁺ interstitials of 6% to 8% were observed regardless of the initial reduction degree of the titania. At the highest copper coverage, the local Ti³⁺ density at the (sub)surface increases to 11.0% and 11.7%. For the SiO_x surface the same could be observed as the Si³⁺/Si⁴⁺ ratio increased from 4% at the lowest copper coverage to 73% at the highest. For the inert SiO_x surface, we suggest an interaction of the copper with defects in the amorphous thin film.