Tin Oxides: Insights into Chemical States from a Nanoparticle Study

Abstract

Tin oxides are semiconductor materials currently attracting close attention in electronics, photovoltaics, gas sensing, and catalysis. Depending on the tin oxidation stateSn­(IV), Sn­(II), or intermediatethe corresponding oxide has either n- or p-type natural conductivity, ascribed to oxygen or metal deficiency in the lattice. Such crystalline imperfections severely complicate the task of establishing tin oxidation state, especially at nanoscale. In spite of the striking differences between SnO2 and SnO in their most fundamental properties, there have been enduring problems in identifying the oxide type. These problems were to a great extent caused by the controversy around the characteristic chemical shift, that is, the difference in electron binding energy of a certain core level in an oxide and its parent metal. Using in situ fabricated bare tin oxide nanoparticles, we have been able to resolve the controversy: Our photoelectron spectroscopic study on tin oxide nanoparticles shows that, in contrast to a common opinion of a close chemical shift for SnO2 and SnO, the shift value for tin­(IV) oxide is, in fact, 3 times larger than that for tin­(II) oxide. Moreover, our investigation of the nanoparticle valence electronic structure clarifies the question of why previously the identification of oxidation states encountered problems.

Bismuth-oxide nanoparticles: study in a beam and as deposited

“…Under our typical fabrication conditions, the metallic shell of various meatl-oxide particles consisted of few monolayers. 13,14,16,17 A similar component distribution could be expected for the Bi-oxide particles created in the present work. As shown in detail in the Results section below, the nanoparticles possessing both metallic and oxidised Bi were formed at the oxygen fraction of ≤2%.…”

“…When the particles have both a metallic and an oxide part, a core-level spectrum of such particles typically contains well-separated responses of both parts. 13,14,16,17 These two parts are in contact so that their Fermi levels are aligned. 24 Then, with both the metal and its oxide responses in one spectrum, the oxide shift can be given a clear definition and can be accurately deduced -as the difference between the core-level energies for the metallic and the oxide parts.…”

“…In the mixed-composition case, the metal and the oxide parts were shown to be radially segregated with the oxide forming the core and the metalthe surface shell. 13,14,16,17 We assigned this segregation and structure to be driven by the relation between the cohesive energies of the metal and its oxide. Another reason for the coreshell geometry is that, in reactive sputtering, the vapour oxidation occurs mostly close to the target where the plasma is localized, and where dissociated and ionized oxygen is produced.…”