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Tin Oxides: Insights into Chemical States from a Nanoparticle Study Charles Wright, Chaofan Zhang, Mikko-Heikki Mikkelä, Erik Mårsell, Anders Mikkelsen, Stacey L. Sorensen, Olle Bjorneholm, and Maxim Tchaplyguine J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05013 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017
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Tin Oxides: Insights into Chemical States from a Nanoparticle Study Charles Wrightj, Chaofan Zhangk, Mikko-Heikki Mikkeläi, Erik Mårsellj, Anders Mikkelsenj, Stacey Sorensenj, Olle Björneholmk, Maxim Tchaplyguinei* j
Synchrotron Radiation Research Division, Department of Physics, Lund University, Box 118,
22100 Lund, Sweden k
Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
i
MAX-lab, Lund University, Box 118, 22100 Lund, Sweden
*e-mail:
[email protected] ACS Paragon Plus Environment
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ABSTRACT: Tin oxides are semiconductor materials currently attracting close attention in electronics, photovoltaics, gas sensing, and catalysis. Depending on 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, the problems caused by the controversy around the characteristic chemical shift − the difference in the 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 spectroscopy 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 Sn(IV)-oxide is, in fact, three times larger than that for Sn(II)-oxide. Moreover, our investigation of the nanoparticle valence electronic structure clarifies the question why previously the identification of the oxidation states encountered problems.
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■ INTRODUCTION
In recent years tin oxides, pure and doped, have been advancing over a broad front in nanoscience and nanotechnology: in optoelectronics1,2,3,4 , photovoltaics5,6,7,8 , in magnetism9,10,11 , in gas sensing12,13,14, in catalysis15,16,17,18. Out of several possible tin oxides the most used one is Sn(IV)-oxide, SnO2, which is a wide-gap semiconductor with natural n-type conductivity. This conductivity has been as a rule assigned to the “effective” oxidation state caused by substantial oxygen deficiency in the crystal lattice10,18,19,20. At the same time the other stable oxide form, Sn(II)-oxide SnO, has p-type conductivity1,3, and metal-atom deficiency has been observed in it21. Also some intermediate/non-stoichiometric tinoxides SnOx, 1
