EXAFS and XANES Study of a Pure and Pd Doped Novel Sn/SnOx Nanomaterial

EXAFS and XANES at the Pd and/or Sn K-edge have been used to characterize pure and palladium doped spherical particles (10?63 nm diameter) of a novel Sn/SnOx nanocomposite. This nanomaterial prepared by decomposition of organic precursors has found application in a new type of high performance solid-state gas sensor. Except for the sample annealed at 600 °C that is mainly pure crystalline tetragonal cassiterite SnO2, the undoped nanoparticles consist of a Sn metallic core surrounded by a layer of tin oxide that is mostly amorphous. In this outer layer, tin atoms were found to be 4?5-fold coordinated to oxygen with bond distances slightly larger than in bulk cassiterite SnO2. The average tin oxidation state in the oxide phase was evaluated using the length of the tin?oxygen bond to be between +3.7 and +3.5. The relative ratio of Sn atoms in the metallic phase was estimated between 15 and 36% of the total number of Sn atoms using the coordination number of the first Sn shell corresponding to the tin ? tetragonal metallic phase. In the palladium doped samples prepared either by codecomposition of Sn and Pd organometallic precursors (volume-doped) or by deposition of palladium on preformed Sn/SnOx nanoparticles (surface-doped) palladium was found surprisingly to be always in a metallic state, and no trace of oxidation of the Pd atoms could be detected. In both samples Pd is surrounded by ca. 2.6 Sn at unusually short distances varying from 2.55 Å in the volume-doped sample to 2.58 Å in the surface-doped sample and by a second coordination shell of ca. 2.3 Pd atoms located at 2.77 Å in the Pd surface-doped sample and only 0.5 Pd atoms in the Pd volume-doped sample. These results show that the volume doping method leads to a more homogeneous mixing of the Pd atoms in the Sn matrix since much fewer Pd atoms were found in the Pd coordination shell. This is consistent both with the large increase in the particle diameter observed by HRTEM and the poor electrical properties that have been measured for the fully oxidized Pd volume-doped materials. In combination with HRTEM, two structural models for the doped materials are discussed:? formation of small metallic Pd platelets around the tin metallic core by complex migration and rearrangement of the Pd atoms and/or formation of a new mixed Pd/Sn phase in the metallic core of the particle