Successful location of tin dopant cations on surface sites of anatase-type TiO2 crystallites evidenced by 119Sn Mössbauer spectroscopic probe and XPS techniques

• H2 annealing Sn4+-doped TiO2 results in the formation of Sn2+ ions on surface sites.
• Under H2 atmosphere, Sn2+ is not reducible to the metallic state up to at least 650 °C.
• Upon exposure to air, Sn2+ is oxidized to form surface-located Sn4+ daughter ion.
• Anatase is shown to allow one to study surface reactions using 119Sn Mossbauer probe.

The present study provides the first experimental evidence for the stabilization of tin dopant cations immediately on the surface of an oxide having a tetragonal structure. 119Sn Mössbauer spectra of the dopant, introduced by air annealing into the bulk of anatase microcrystals, showed that it was located, in the tetravalent state, in somewhat distorted octahedral sites of a unique type. On the contrary, the reduced tin species, formed upon subsequent hydrogen annealing the Sn4+-doped samples, are found to occupy different sites being characterized by two sets of the isomer shift δ and quadrupole splitting ΔEQ values (δI = 3.25 mm s−1, ΔEQI = 1.75 mm s−1; and δII = 2.85 mm s−1, ΔEQII = 1.71 mm s−1). Either of them implies both the divalent state of tin atoms and their presence at low-coordination sites that can be assigned to the surface of crystallites. Mössbauer spectra of Sn4+←2+ daughter ions, formed upon contact with air of Sn2+, consist of a symmetrically broadened peak characterized by only slightly different average values of both the isomer shift (<δ> = 0.07 mm s−1) and quadrupole splitting (<ΔEQ> = 0.50 mm s−1), as compared to the δ and ΔEQ values for the bulk-located Sn4+. However, considerable broadening of Sn4+←2+ doublet components (Γ = 0.97 mm s−1) allows one to suggest that these secondary formed ions remain distributed over the non equivalent sites inherited from their Sn2+ precursors. The occurrence of Sn4+←2+ at surface sites is independently proven by XPS measurements that revealed a greater than 10-fold enrichment with tin of 3–5 nm thick surface layers.

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