4.6 Article

Spectroscopic assessment of the role of hydrogen in surface defects, in the electronic structure and transport properties of TiO2, ZnO and SnO2 nanoparticles

期刊

PHYSICAL CHEMISTRY CHEMICAL PHYSICS
卷 15, 期 5, 页码 1417-1430

出版社

ROYAL SOC CHEMISTRY
DOI: 10.1039/c2cp42601c

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资金

  1. Korean-Swiss Cooperative Program in Science and Technology Project Spectroscopy on Photoelectrochemical Electrode Materials (SOPEM)
  2. Swiss State Secretary for Education and Research (SBF)
  3. European Union Regional Development Programme for Technological Development [POIG.01.03.01-02-002/08-00]
  4. PhD School Switzerland-Poland
  5. Swiss National Science Foundation [132126]
  6. National Research Council of Science & Technology (NST), Republic of Korea [C32250] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)

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The interaction of metal oxides with gases is very important for the operation of energy devices such as fuel cells and gas sensors, and also relevant for materials synthesis and processing. The electronic transport properties of metal oxides for the aforementioned devices strongly depend on the chemistry of these gases and on the presence or absence of defects on the surface and in the bulk. The Debye screening length is in this respect a material specific property which becomes particularly significant when the material is comprised of nanoparticles. In the present study, poly-crystalline TiO2, ZnO and SnO2 nanoparticles were synthesized by a high temperature flame spray combustion process and investigated for their interaction with hydrogen. The chemistry of the reduced and oxidized surfaces of these metal oxides, where the interaction with gases takes place, was investigated in detail with X-ray photoelectron spectroscopy (XPS). The transitions found near E-F with XPS are consistent with those found by diffuse reflectance spectroscopy (DRS) and were assigned to surface states originating from non-equilibrium oxygen vacancies. We show how the non-stoichiometric character of the metal oxide surface constitutes electronic surface defects, in particular oxygen vacancies which allow for additional transitions near the Fermi energy (E-F). The concentration of these surface defects can be strongly reduced by thermal after-treatment under air or increased by Ar+-sputtering, after which significant spectral features appear near E-F. Such prominent changes are particularly observed for TiO2 and SnO2, whereas the stoichiometry of the ZnO surface seems to be less responsive to such reducing and oxidizing conditions. Pronounced changes of the electrical conductivity upon changing from reducing to oxidizing conditions at elevated temperatures were monitored by electrochemical impedance spectroscopy (EIS). The lowering of the potential barrier for the charge transport particularly at lower temperatures already at reducing conditions is confirmed. The impedance response indicates that charge transfer is governed predominantly by one single process, i.e. by interaction of surface-like states localized within depletion layer with gas molecules. This implies that the free charge carriers in the material are determined by the intrinsic property like non-stoichiometry. Gas sensors made from such FSS nanoparticulate metal oxides showed well developed sensing characteristics of the hydrogen sensing at moderate temperatures.

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