Nanoscale insights into doping behavior, particle size and surface effects in trivalent metal doped SnO2 (original) (raw)

Despite considerable research, the location of an aliovalent dopant into SnO 2 nanoparticles is far to be clarified. The aim of the present study on trivalent lanthanide doped SnO 2 is to differentiate between substitutional versus interstitial and surface versus bulk doping, delineate the bulk and surface defects induced by doping and establish an intrinsic dopant distribution. We evidence for the first time a complex distribution of intrinsic nature composed of substitutional isolated, substitutional associates with defects as well as surface centers. Such multi-modal distribution is revealed for Eu and Sm, while Pr, Tb and Dy appear to be distributed mostly on the SnO 2 surface. Like the previously reported case of Eu, Sm displays a long-lived luminescence decaying in the hundreds of ms scale which is likely related to a selective interaction between the traps and the substitutional isolated center. Analyzing the time-gated luminescence, we conclude that the local lattice environment of the lattice Sn is not affected by the particle size, being remarkably similar in the ~2 and 20 nm particles. The photocatalytic measurements employed as a probe tool confirm the conclusions from the luminescence measurements concerning the nature of defects and the temperature induced migration of lanthanide dopants. There has been considerable research over the past decades on the n-type wide band gap metal oxide semiconductor, tin oxide (SnO 2) due to its broad spectrum of applications. It is commonly used in transparent conducting electrodes and chemical sensors 1, 2 production of batteries in conjunction with carbon based materials 3 , photocatalysts either in pure state, doped with non-lanthanide 4 , lanthanide ions (Ln) 5 or in combination with another oxide (for example SnO 2 /TiO 2 6 , or SnO 2 /ZnO 7) as well as photocatalysts with a post-illumination photocatalytic "memory" 8. SnO 2 has the rutile-type tetragonal structure belonging to the P 42 /mnm space group (lattice parameters a = b = 4.738 Å and c = 3.187 Å) with a band energy-gap situated between 3.5 and 3.8 eV according to both experimental results and theoretical calculations 9, 10. Band-gap engineering has been used as an effective way to tune the band structure and optoelectronic properties of this oxide 11. For this purpose, SnO 2 has been synthetized by exploiting numerous approaches such as precipitation 12 , photochemical growth at the air-water interface 9, 10 , thermal decomposition 13 , sol-gel 14 , surfactant-assisted solvothermal 10 , hydrothermal synthesis 15, 16 and sono-chemical method 17. Doping of SnO 2 nanomaterials with metal cations proved to be a successful tool for tailoring their electrical, optical, and microstructural properties. The luminescence of pure SnO 2 , observed in the UV and/or visible region (350-550 nm) is generally correlated with the presence of crystalline defects resulting from the various synthesis processes 18, 19. The literature agrees towards the oxygen vacancies as the most probable candidates for the recombination centers in the emission processes of SnO 2 18, 19. Of the various metal dopants of SnO 2 , the aliovalent Ce 3+ 18, 19 , Mn 2+ 18, 19 Co 2+ 20 , Ni 2+ 21 or Cr 3+ 22 revealed significant information on the relationships between doping, defects related luminescence, surface effects, changes in morphology and particle size.